fix(web): post-review fixes for source tabs

- Reset per-platform fields on fetch failure (was leaving stale data
  for up to 5 minutes after the user logged out of NetEase).
- Extract availableSources getter on the store; deduplicate the daily/
  user availability computeds in Home.vue and Library.vue.
- Add comment explaining why recommendAvailable always seeds netease.
- Import Source type in SourceTabs.vue from the store instead of
  redeclaring locally, removing a future type-drift risk.

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

web/src/components/SourceTabs.vue

@@ -14,7 +14,7 @@
 </template>
 
 <script setup lang="ts">
-type Source = 'netease' | 'qq';
+import type { Source } from '../stores/player.js';
 
 const LABELS: Record<Source, string> = {
   netease: '网易云',