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/channels.json

@@ -13,6 +13,7 @@
   },
   "tags": {
     "yourTags": "Your tags",
+    "manage": "Manage tags",
     "yourTagsHint": "Your personal labels. Attach them to channels below, then filter the feed by tag from the sidebar. (Separate from the automatic language/topic tags.)",
     "newTag": "new tag",
     "createTag": "Create tag"
@@ -56,6 +57,7 @@
     "subs": "{{formatted}} subs",
     "openOnYouTube": "Open on YouTube",
     "editTags": "Edit tags",
+    "filterFeedByTag": "Filter the feed by “{{name}}”",
     "priorityHint": "Your ranking for this channel. Sort the feed by “Channel priority” to bring higher-priority channels to the top.",
     "raisePriority": "Raise priority",
     "lowerPriority": "Lower priority",
@@ -94,6 +96,7 @@
     "fullHistoryFailed": "Couldn't request full history",
     "needYouTube": "Connect your YouTube account to do this.",
     "connect": "Connect",
+    "filteredByTag": "Feed filtered by tag: {{name}}",
     "resetDone": "Channel reset — re-fetching now",
     "resetFailed": "Couldn't reset this channel"
   },