fix(flutter): seed connectivityProvider with initial value (unhang detail screens)

Root cause for "only the first tile loads, others spin forever":

Connectivity().onConnectivityChanged only emits on connectivity
*changes*, not on subscription. Cold-cache code paths in albumProvider,
artistProvider, playlistsProvider, likes etc. all gate their fetches
on `await ref.read(connectivityProvider.future)`. Until the OS
happened to report the first connectivity flip, that future never
resolved — so any detail screen for an album/artist/playlist not
already in the drift cache hung indefinitely at the connectivity
check.

The "first tile" appeared to work because by the time the user
tapped it, a connectivity event had landed (or that album was already
cached from a prior session). Subsequent tiles whose data wasn't yet
cached blocked.

Switch the provider to async* and seed it with an immediate
checkConnectivity() before forwarding the change stream. .future
now resolves on first read regardless of whether the OS has reported
a change yet.

flutter_client/lib/cache/connectivity_provider.dart

@@ -5,9 +5,18 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 /// connectivity_plus reports the union (wifi, mobile, vpn, etc.) — operator
 /// chose "no Wi-Fi gate" for #357, so any connection means "go ahead and
 /// pull/cache".
-final connectivityProvider = StreamProvider<bool>((ref) {
+///
+/// IMPORTANT: onConnectivityChanged only emits on *changes*, not on
+/// subscription. Without seeding an initial value via checkConnectivity(),
+/// any consumer using `ref.read(connectivityProvider.future)` would
+/// block until the OS happened to report a connectivity flip — which
+/// is exactly what made album/artist/playlist detail screens spin
+/// forever for tiles tapped before the first event landed.
+final connectivityProvider = StreamProvider<bool>((ref) async* {
   final c = Connectivity();
-  return c.onConnectivityChanged.map(
-    (results) => results.any((r) => r != ConnectivityResult.none),
-  );
+  bool isOnline(List<ConnectivityResult> rs) =>
+      rs.any((r) => r != ConnectivityResult.none);
+
+  yield isOnline(await c.checkConnectivity());
+  yield* c.onConnectivityChanged.map(isOnline);
 });