feat: Add persistent contacts cache for @mention autocomplete

Contacts cache accumulates all known node names from device contacts
and adverts into a JSONL file, so @mentions work even after contacts
are removed from the device. Background thread scans adverts every
45s and parses advert payloads to extract public keys and node names.

Existing Contacts page now shows merged view with "Cache" badge for
contacts not on device, plus source filter (All/On device/Cache only).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/main.py

@@ -16,6 +16,7 @@ from app.routes.api import api_bp
 from app.version import VERSION_STRING, GIT_BRANCH
 from app.archiver.manager import schedule_daily_archiving
 from app.meshcore.cli import fetch_device_name_from_bridge
+from app.contacts_cache import load_cache, scan_new_adverts, initialize_from_device
 
 # Commands that require longer timeout (in seconds)
 SLOW_COMMANDS = {
@@ -108,6 +109,34 @@ def create_app():
 
     threading.Thread(target=init_device_name, daemon=True).start()
 
+    # Background thread: contacts cache initialization and periodic advert scanning
+    def init_contacts_cache():
+        # Wait for device name to resolve
+        time.sleep(10)
+
+        cache = load_cache()
+
+        # Seed from device contacts if cache is empty
+        if not cache:
+            try:
+                from app.routes.api import get_contacts_detailed_cached
+                success, contacts, error = get_contacts_detailed_cached()
+                if success and contacts:
+                    initialize_from_device(contacts)
+                    logger.info("Contacts cache seeded from device")
+            except Exception as e:
+                logger.error(f"Failed to seed contacts cache: {e}")
+
+        # Periodic advert scan loop
+        while True:
+            time.sleep(45)
+            try:
+                scan_new_adverts()
+            except Exception as e:
+                logger.error(f"Contacts cache scan error: {e}")
+
+    threading.Thread(target=init_contacts_cache, daemon=True).start()
+
     logger.info(f"mc-webui started - device: {config.MC_DEVICE_NAME}")
     logger.info(f"Messages file: {config.msgs_file_path}")
     logger.info(f"Serial port: {config.MC_SERIAL_PORT}")