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/templates/contacts-existing.html

@@ -29,6 +29,13 @@
 
     <!-- Filter and Sort Toolbar -->
     <div class="filter-sort-toolbar">
+        <!-- Source Filter -->
+        <select class="form-select" id="sourceFilter">
+            <option value="ALL">All sources</option>
+            <option value="DEVICE">On device</option>
+            <option value="CACHE">Cache only</option>
+        </select>
+
         <!-- Type Filter -->
         <select class="form-select" id="typeFilter">
             <option value="ALL">All Types</option>