feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

ui-electrobun/src/mainview/TaskBoardTab.svelte

@@ -168,14 +168,24 @@
     dragOverCol = null;
   }
 
-  // ── Init + polling ───────────────────────────────────────────────────
+  // ── Init + event-driven updates (Feature 4) ─────────────────────────
 
   let pollTimer: ReturnType<typeof setInterval> | null = null;
 
+  // Feature 4: Listen for push events, fallback to 30s poll
+  function onTaskChanged(payload: { groupId: string }) {
+    if (!payload.groupId || payload.groupId === groupId) loadTasks();
+  }
+
   $effect(() => {
     loadTasks();
-    pollTimer = setInterval(loadTasks, 5000);
-    return () => { if (pollTimer) clearInterval(pollTimer); };
+    appRpc.addMessageListener('bttask.changed', onTaskChanged);
+    // Feature 4: Fallback 30s poll for missed events
+    pollTimer = setInterval(loadTasks, 30000);
+    return () => {
+      if (pollTimer) clearInterval(pollTimer);
+      appRpc.removeMessageListener?.('bttask.changed', onTaskChanged);
+    };
   });
 </script>