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/lib/sidebarLayout.ts

@@ -4,14 +4,15 @@
 
 // `show`, `sort` and `content` moved to the feed toolbar (above the cards); they are no
 // longer sidebar widgets. normalizeLayout drops them from any persisted layout automatically.
-export type WidgetId = "date" | "language" | "topic";
+export type WidgetId = "date" | "language" | "topic" | "tags";
 
-export const ALL_WIDGETS: WidgetId[] = ["date", "language", "topic"];
+export const ALL_WIDGETS: WidgetId[] = ["date", "language", "topic", "tags"];
 
 export const WIDGET_TITLES: Record<WidgetId, string> = {
   date: "Upload date",
   language: "Language",
   topic: "Topic",
+  tags: "Tags",
 };
 
 export interface SidebarLayout {