feat(tags): sync the sidebar tag dropdown with the shared filter state

Writable $derived instead of local $state, so clicking a tag chip
elsewhere (the Tags panel) is reflected here too, not just changes
made directly in this dropdown.

src/lib/views/MeetingsList.svelte

@@ -17,7 +17,10 @@
   // Tag/date filters (T8.3, FR-SEARCH-2) apply to the plain list, not
   // full-text search — changing one drops out of search mode so the
   // filtered list is immediately visible rather than hidden behind results.
-  let tagFilter = $state("");
+  // Writable derived (not $state+$effect) so this dropdown also reflects a
+  // tag filter triggered elsewhere (e.g. clicking a chip in the Tags panel),
+  // while still being directly editable via bind:value below.
+  let tagFilter = $derived(meetings.filter.tag ?? "");
   let fromFilter = $state("");
   let toFilter = $state("");