feat(search): add global message search with FTS5 backend

- New GET /api/messages/search endpoint using existing FTS5 indexes
- Search modal accessible from navbar search icon
- Debounced search (300ms) across all channel and DM messages
- Results show source (channel/DM), sender, timestamp with highlights
- Click result navigates to the relevant channel or DM conversation

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

app/static/css/style.css

@@ -1402,3 +1402,26 @@ main {
         font-size: 0.9rem;
     }
 }
+
+/* =============================================================================
+   Global Search
+   ============================================================================= */
+
+#searchResults {
+    max-height: 60vh;
+    overflow-y: auto;
+}
+
+#searchResults .list-group-item {
+    border-left: 3px solid transparent;
+}
+
+#searchResults .list-group-item:hover {
+    border-left-color: var(--bs-primary);
+}
+
+#searchResults mark {
+    background-color: #fff3cd;
+    padding: 0 2px;
+    border-radius: 2px;
+}