fix: address all known follow-up issues except NetEase batch precheck

Surface failures + tighten edges:

1. Toast for /play-song & /play-playlist failures. Backend now returns
   {ok, message} (localized in Chinese to match the rest of the UI).
   Store stashes a notification on ok=false; new Toast.vue mounted in
   App.vue shows it for 3-5s then fades. Clicking the X dismisses
   immediately. Sits above the player on desktop and above the mobile
   tabbar on phones.

2. QQ collected playlists pagination. fcg_get_profile_order_asset.fcg
   returns max 30 per call; we now loop using has_more / short-page
   detection up to a 300-playlist hard cap. Single-call users (typical)
   exit the loop on the first iteration so no extra requests.

3. getPlayableSongIds chunking. 100 mids per request keeps URL well
   under 8KB; chunk-level errors are isolated so a transient blip on
   one chunk doesn't poison the whole batch. Returns null only when
   every chunk failed (caller falls back to sequential retry).

4. SourceTabs single-source mode now renders a small subdued "网易云"
   or "QQ" label instead of vanishing entirely, so the user always
   knows which platform's data they're looking at.

5. Hide the "我的歌单 N" count badge when N=0 — Home and Library no
   longer show "我的歌单 0" with an empty grid.

6. Auth state change invalidates fetchHomeData cache. Previously, a
   user who logged out as account A and into account B within 5
   minutes would see A's playlists. Now we always re-check auth at
   the top of fetchHomeData and bypass the TTL cache when authStatus
   has changed.

Out of scope:
- NetEase analogous batch precheck (per request).
- 60s TS3 UDP idle disconnect — that's the bundled @honeybbq/teamspeak-
  client UDP layer kicking when no server packet arrives in 60s. It's
  baked in (constant `v=6e4`) and not exposed as an option, and root
  cause is server-side or network-layer behavior we can't reach from
  here.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/components/SourceTabs.vue

@@ -1,15 +1,20 @@
 <template>
-  <div v-if="sources.length >= 2" class="source-tabs">
-    <button
-      v-for="src in sources"
-      :key="src"
-      type="button"
-      class="source-tab"
-      :class="{ active: src === modelValue }"
-      @click="$emit('update:modelValue', src)"
-    >
-      {{ LABELS[src] }}
-    </button>
+  <div v-if="sources.length > 0" class="source-tabs">
+    <template v-if="sources.length >= 2">
+      <button
+        v-for="src in sources"
+        :key="src"
+        type="button"
+        class="source-tab"
+        :class="{ active: src === modelValue }"
+        @click="$emit('update:modelValue', src)"
+      >
+        {{ LABELS[src] }}
+      </button>
+    </template>
+    <span v-else class="source-tab-label" :title="`数据来自${LABELS[sources[0]]}`">
+      {{ LABELS[sources[0]] }}
+    </span>
   </div>
 </template>
 
@@ -63,6 +68,17 @@ defineEmits<{
   }
 }
 
+// Single-source mode: not interactive, but tells the user which platform
+// they're looking at instead of leaving them guessing.
+.source-tab-label {
+  padding: 4px 10px;
+  font-size: var(--fs-xs);
+  font-weight: var(--fw-medium);
+  color: var(--text-tertiary);
+  background: var(--hover-bg);
+  border-radius: var(--radius-sm);
+}
+
 @media (max-width: 768px) {
   .source-tabs {
     margin-left: 8px;