fix(flutter): album/artist detail covers + cacheFirst tracing for hangs

Album detail screen rendered the cover slot as a bare slate Container
— no actual image. Wire ServerImage at /api/albums/<id>/cover (96dp,
rounded). Same for artist detail: ClipOval around ServerImage of the
artist's coverUrl, falling back to slate when empty.

Artist navigation hang: artistProvider goes through cacheFirst, which
had no logging — so we couldn't see whether the cold-cache fetch was
running. Add an optional `tag` parameter that, when set, debugPrints
each step (drift hit/miss, connectivity, fetch start/done/error).
Wire tags into artistProvider and artistAlbumsProvider, plus add the
3s connectivity timeout there too as belt-and-suspenders.

Next test pass should show cacheFirst[artist(<id>)]: lines that
pinpoint where the artist screen is sticking.

flutter_client/lib/cache/cache_first.dart

@@ -17,6 +17,8 @@
 
 import 'dart:async';
 
+import 'package:flutter/foundation.dart' show debugPrint;
+
 /// Wraps the watch + cold-cache fallback pattern. Generic over:
 ///   D — the drift row type (or TypedResult for joins)
 ///   T — the result type the caller wants (e.g. `List<ArtistRef>`)
@@ -35,14 +37,19 @@ Stream<T> cacheFirst<D, T>({
   required T Function(List<D>) toResult,
   required Future<bool> Function() isOnline,
   bool alwaysRefresh = false,
+  String? tag,
 }) async* {
   // Tracks whether we've already kicked off a stale-while-revalidate
   // refresh for this stream subscription, so we don't fire one on every
   // drift re-emission (otherwise the populate cycles forever).
   var revalidated = false;
+  void log(String msg) {
+    if (tag != null) debugPrint('cacheFirst[$tag]: $msg');
+  }
 
   await for (final rows in driftStream) {
     if (rows.isNotEmpty) {
+      log('drift hit (${rows.length} rows)');
       yield toResult(rows);
       // Stale-while-revalidate: yield cache immediately, then kick off
       // a REST refresh in the background. Drift watch() picks up the
@@ -55,15 +62,20 @@ Stream<T> cacheFirst<D, T>({
       }
       continue;
     }
+    log('drift miss; checking connectivity');
     if (await isOnline()) {
+      log('online; calling fetchAndPopulate');
       try {
         await fetchAndPopulate();
+        log('fetchAndPopulate done; awaiting drift re-emit');
         // The drift watch() stream re-emits with the populated rows on
         // the next loop iteration. Don't yield here.
-      } catch (_) {
+      } catch (e, st) {
+        log('fetchAndPopulate failed: $e\n$st');
         yield toResult(rows); // empty result; caller surfaces error
       }
     } else {
+      log('offline; yielding empty');
       yield toResult(rows); // empty result; offline
     }
   }