feat(tags): tag UX overhaul + per-card video reset

- Channel-row tags show only what's attached; a '+' opens a per-channel picker. Clicking a tag
  filters the feed by it; a 'Your tags' sidebar widget makes that filter visible/clearable.
- Tag manager dialog (add/rename/delete; delete only confirms when the tag is in use), reachable
  from the Channel manager and the feed sidebar; hovering a tag's count lists its channels, each a
  link that focuses it in the Channel manager (DataTable gains an external filter input).
- Video cards get a reset action that clears all watch state (incl. an in-progress position).
- api.req() now raises the error dialog on 5xx and 400/409/422 with the server's reason.

frontend/src/i18n/locales/de/sidebar.json

@@ -26,6 +26,7 @@
   "clearDates": "Daten löschen",
   "reshuffle": "Neu mischen",
   "noMatchingTags": "Keine passenden Tags",
+  "manageTags": "Verwalten",
   "shareView": "Link zu dieser Ansicht kopieren",
   "shareCopied": "Ansichts-Link in die Zwischenablage kopiert",
   "shareFailed": "Link konnte nicht kopiert werden",
@@ -35,7 +36,8 @@
     "date": "Upload-Datum",
     "content": "Inhaltstyp",
     "language": "Sprache",
-    "topic": "Thema"
+    "topic": "Thema",
+    "tags": "Deine Tags"
   },
   "show": {
     "unwatched": "Ungesehen",