feat(track): icon-based status badges, detect locally-cached tracks

Replace the labelled availability/metadata badges in track rows with
small icon+tooltip indicators (cloud/hard-drives/warning/etc, derived
from TrackAvailability and MetadataStatus).

Add a `connection` slice fed by a single status poller (Sidebar) so
other components can cheaply check backend reachability. TrackRow uses
this plus the offline audio cache to show "Local" instead of a stale
"On server" when the backend is down but the track is already cached.

src/hooks/useConnectionStatus.ts

@@ -1,7 +1,12 @@
 import { useState, useEffect } from 'react';
 import { getApiBaseUrl } from '../config/runtime-config';
+import { useAppDispatch, useAppSelector } from './useAppDispatch';
+import {
+  setConnectionStatus,
+  type ConnectionStatus,
+} from '../store/slices/connection';
 
-type ConnectionStatus = 'connected' | 'connecting' | 'disconnected' | 'error';
+export type { ConnectionStatus };
 
 /** Pings `${baseUrl}/health` (defaults to the active instance's base URL). */
 export function useConnectionStatus(baseUrl?: string) {
@@ -36,3 +41,26 @@ export function useConnectionStatus(baseUrl?: string) {
 
   return status;
 }
+
+/**
+ * Like `useConnectionStatus`, but also mirrors the result into the
+ * `connection` slice so other components can read the active instance's
+ * reachability via `useIsOffline` without running their own poller.
+ * Mount once (in `Sidebar`, which lives for the app's whole lifetime).
+ */
+export function useConnectionStatusSync(): ConnectionStatus {
+  const status = useConnectionStatus();
+  const dispatch = useAppDispatch();
+
+  useEffect(() => {
+    dispatch(setConnectionStatus(status));
+  }, [status, dispatch]);
+
+  return status;
+}
+
+/** Whether the active backend instance is currently unreachable. */
+export function useIsOffline(): boolean {
+  const status = useAppSelector((s) => s.connection.status);
+  return status === 'disconnected' || status === 'error';
+}