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/components/common/Icon.tsx

@@ -15,6 +15,7 @@ import {
   ArrowsClockwise,
   CheckCircle,
   Cloud,
+  CloudSlash,
   DotsSixVertical,
   GearSix,
   HardDrives,
@@ -75,6 +76,7 @@ const ICONS = {
   'speaker-high': SpeakerHigh,
   'speaker-x': SpeakerSimpleX,
   cloud: Cloud,
+  'cloud-slash': CloudSlash,
   'check-circle': CheckCircle,
   'warning-circle': WarningCircle,
   'sign-out': SignOut,