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/en/sidebar.json

@@ -26,6 +26,7 @@
   "clearDates": "clear dates",
   "reshuffle": "Reshuffle",
   "noMatchingTags": "No matching tags here",
+  "manageTags": "Manage",
   "shareView": "Copy a link to this view",
   "shareCopied": "View link copied to clipboard",
   "shareFailed": "Couldn't copy the link",
@@ -35,7 +36,8 @@
     "date": "Upload date",
     "content": "Content type",
     "language": "Language",
-    "topic": "Topic"
+    "topic": "Topic",
+    "tags": "Your tags"
   },
   "show": {
     "unwatched": "Unwatched",