feat: add observer multi-select and collapsible filters to list pages

- Add observer multi-select (<select multiple size=2>) to Advertisements and
  Messages filter bars, populated from /api/v1/nodes?observer=true
- Make all filter sections collapsible via <details> on Nodes, Advertisements,
  and Messages pages; collapsed by default, auto-expands when active filters
  exist, preserves open state across auto-refresh ticks
- Add backend observer=true|false query param to GET /api/v1/nodes for
  observer-only or non-observer-only node filtering via subquery
- Change observed_by in Advertisements/Messages API from single public_key
  to list[str] with .in_() for multi-select support
- Fix router.js and api.js to handle array query params (duplicate keys
  promoted to arrays, .append() per element)
- Fix createFilterHandler to use FormData.getAll() for multi-value support
- Replace DaisyUI form-control/label/label-text classes with Tailwind-native
  equivalents (flex flex-col gap-1, flex items-center py-1, opacity-80 text-sm)
  since DaisyUI CSS is tree-shaken from the build output
- Thicker collapsible border (border-2 border-base-content/25) visible in
  both light and dark themes
- Bottom-align Filter/Clear buttons via two-row form layout
- Move Observer filter to last position on Advertisements page
- Add filter_observer_label i18n key
- Add tests for observer=true node filtering and multi observer params

tests/test_api/test_advertisements.py

@@ -84,22 +84,6 @@ class TestListAdvertisements:
         assert len(data["items"]) == 1
         assert data["items"][0]["node_tag_name"] == "Friendly Search Name"
 
-    def test_list_advertisements_filter_by_public_key(
-        self, client_no_auth, sample_advertisement
-    ):
-        """Test filtering advertisements by public key."""
-        response = client_no_auth.get(
-            f"/api/v1/advertisements?public_key={sample_advertisement.public_key}"
-        )
-        assert response.status_code == 200
-        data = response.json()
-        assert len(data["items"]) == 1
-
-        response = client_no_auth.get("/api/v1/advertisements?public_key=nonexistent")
-        assert response.status_code == 200
-        data = response.json()
-        assert len(data["items"]) == 0
-
 
 class TestGetAdvertisement:
     """Tests for GET /advertisements/{id} endpoint."""
@@ -199,14 +183,14 @@ class TestListAdvertisementsFilters:
         data = response.json()
         assert len(data["items"]) == 1
 
-    def test_filter_by_observed_by(
+    def test_list_advertisements_filter_by_observed_by_single(
         self,
         client_no_auth,
         sample_advertisement,
         sample_advertisement_with_receiver,
         receiver_node,
     ):
-        """Test filtering advertisements by receiver node."""
+        """Test filtering advertisements by a single receiver node."""
         response = client_no_auth.get(
             f"/api/v1/advertisements?observed_by={receiver_node.public_key}"
         )
@@ -214,6 +198,57 @@ class TestListAdvertisementsFilters:
         data = response.json()
         assert len(data["items"]) == 1
 
+    def test_list_advertisements_filter_by_observed_by_multiple(
+        self,
+        client_no_auth,
+        api_db_session,
+        receiver_node,
+    ):
+        """Test filtering advertisements by multiple receiver nodes."""
+        # Create second receiver node
+        second_receiver = receiver_node.__class__(
+            public_key="2nd1232nd1232nd1232nd1232nd1232n",
+            name="SecondObserver",
+            first_seen=datetime.now(timezone.utc),
+        )
+        api_db_session.add(second_receiver)
+        api_db_session.commit()
+
+        # Create two advertisements, each observed by a different receiver
+        ad1 = Advertisement(
+            public_key="ad1pubad1pubad1pubad1pubad1pubad",
+            name="AD1",
+            adv_type="CLIENT",
+            received_at=datetime.now(timezone.utc),
+            observer_node_id=receiver_node.id,
+        )
+        ad2 = Advertisement(
+            public_key="ad2pubad2pubad2pubad2pubad2pubad",
+            name="AD2",
+            adv_type="CLIENT",
+            received_at=datetime.now(timezone.utc),
+            observer_node_id=second_receiver.id,
+        )
+        api_db_session.add_all([ad1, ad2])
+        api_db_session.commit()
+
+        # Filter by both receivers
+        response = client_no_auth.get(
+            f"/api/v1/advertisements?observed_by={receiver_node.public_key}&observed_by={second_receiver.public_key}"
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data["items"]) == 2
+
+        # Filter by just the first receiver
+        response = client_no_auth.get(
+            f"/api/v1/advertisements?observed_by={receiver_node.public_key}"
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data["items"]) == 1
+        assert data["items"][0]["name"] == "AD1"
+
     def test_filter_by_since(self, client_no_auth, api_db_session):
         """Test filtering advertisements by since timestamp."""
         now = datetime.now(timezone.utc)

tests/test_api/test_messages.py

@@ -216,14 +216,14 @@ class TestListMessagesFilters:
         data = response.json()
         assert len(data["items"]) == 0
 
-    def test_filter_by_observed_by(
+    def test_filter_by_observed_by_single(
         self,
         client_no_auth,
         sample_message,
         sample_message_with_receiver,
         receiver_node,
     ):
-        """Test filtering messages by receiver node."""
+        """Test filtering messages by a single receiver node."""
         response = client_no_auth.get(
             f"/api/v1/messages?observed_by={receiver_node.public_key}"
         )
@@ -232,6 +232,57 @@ class TestListMessagesFilters:
         assert len(data["items"]) == 1
         assert data["items"][0]["text"] == sample_message_with_receiver.text
 
+    def test_filter_by_observed_by_multiple(
+        self,
+        client_no_auth,
+        api_db_session,
+        receiver_node,
+    ):
+        """Test filtering messages by multiple receiver nodes."""
+        # Create second receiver node
+        second_receiver = Node(
+            public_key="2ndmsg2ndmsg2ndmsg2ndmsg2ndmsg2n",
+            name="SecondMsgObserver",
+            first_seen=datetime.now(timezone.utc),
+        )
+        api_db_session.add(second_receiver)
+        api_db_session.commit()
+
+        # Create two messages, each observed by a different receiver
+        msg1 = Message(
+            message_type="channel",
+            channel_idx=1,
+            text="Msg from receiver A",
+            received_at=datetime.now(timezone.utc),
+            observer_node_id=receiver_node.id,
+        )
+        msg2 = Message(
+            message_type="channel",
+            channel_idx=2,
+            text="Msg from receiver B",
+            received_at=datetime.now(timezone.utc),
+            observer_node_id=second_receiver.id,
+        )
+        api_db_session.add_all([msg1, msg2])
+        api_db_session.commit()
+
+        # Filter by both receivers
+        response = client_no_auth.get(
+            f"/api/v1/messages?observed_by={receiver_node.public_key}&observed_by={second_receiver.public_key}"
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data["items"]) == 2
+
+        # Filter by just the first receiver
+        response = client_no_auth.get(
+            f"/api/v1/messages?observed_by={receiver_node.public_key}"
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data["items"]) == 1
+        assert data["items"][0]["text"] == "Msg from receiver A"
+
     def test_filter_by_since(self, client_no_auth, api_db_session):
         """Test filtering messages by since timestamp."""
         now = datetime.now(timezone.utc)

tests/test_api/test_nodes.py

@@ -168,6 +168,44 @@ class TestListNodesFilters:
         assert room_node.public_key in room_keys
         assert name_only_room_node.public_key not in room_keys
 
+    def test_filter_by_observer_true(
+        self, client_no_auth, api_db_session, receiver_node
+    ):
+        """Test filtering nodes that have observed events."""
+        from datetime import datetime, timezone
+
+        from meshcore_hub.common.models import Advertisement, Message
+
+        # This node has observed an ad and a message
+        advert = Advertisement(
+            public_key="obsflt1obsflt1obsflt1obsflt1ob",
+            name="ObservedAd",
+            adv_type="CLIENT",
+            received_at=datetime.now(timezone.utc),
+            observer_node_id=receiver_node.id,
+        )
+        msg = Message(
+            message_type="channel",
+            channel_idx=1,
+            text="Observed msg",
+            received_at=datetime.now(timezone.utc),
+            observer_node_id=receiver_node.id,
+        )
+        api_db_session.add_all([advert, msg])
+        api_db_session.commit()
+
+        response = client_no_auth.get("/api/v1/nodes?observer=true")
+        assert response.status_code == 200
+        data = response.json()
+        observer_keys = {item["public_key"] for item in data["items"]}
+        assert receiver_node.public_key in observer_keys
+
+        response = client_no_auth.get("/api/v1/nodes?observer=false")
+        assert response.status_code == 200
+        data = response.json()
+        non_observer_keys = {item["public_key"] for item in data["items"]}
+        assert receiver_node.public_key not in non_observer_keys
+
 
 class TestGetNode:
     """Tests for GET /nodes/{public_key} endpoint."""