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/store/slices/connection.ts

@@ -0,0 +1,28 @@
+import { createSlice, type PayloadAction } from '@reduxjs/toolkit';
+
+export type ConnectionStatus =
+  | 'connected'
+  | 'connecting'
+  | 'disconnected'
+  | 'error';
+
+export interface ConnectionState {
+  status: ConnectionStatus;
+}
+
+export const connectionInitialState: ConnectionState = {
+  status: 'connecting',
+};
+
+export const connectionSlice = createSlice({
+  name: 'connection',
+  initialState: connectionInitialState,
+  reducers: {
+    setConnectionStatus(state, action: PayloadAction<ConnectionStatus>) {
+      state.status = action.payload;
+    },
+  },
+});
+
+export const { setConnectionStatus } = connectionSlice.actions;
+export default connectionSlice.reducer;