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/styles/shell.css

@@ -435,6 +435,14 @@
   animation-play-state: paused;
   height: 100%;
 }
+.spin {
+  animation: spin 1.2s linear infinite;
+}
+@keyframes spin {
+  to {
+    transform: rotate(360deg);
+  }
+}
 @keyframes playing-bar-bounce {
   0%,
   100% {