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/card.json

@@ -8,6 +8,7 @@
   "unhide": "Einblenden",
   "hide": "Ausblenden",
   "onlyThisChannel": "Nur dieser Kanal",
+  "resetState": "Zurücksetzen — Fortschritt/Status löschen",
   "thisChannel": "Dieser Kanal",
   "continueTitle": "Dort fortsetzen, wo du aufgehört hast",
   "continue": "Fortsetzen",