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/components/DataTable.tsx

@@ -66,6 +66,7 @@ export default function DataTable<T>({
   rowClassName,
   controlsPosition = "bottom",
   controlsLeading,
+  externalFilter,
 }: {
   rows: T[];
   columns: Column<T>[];
@@ -77,6 +78,9 @@ export default function DataTable<T>({
   rowClassName?: (row: T) => string;
   controlsPosition?: "top" | "bottom" | "both";
   controlsLeading?: ReactNode;
+  // Set a column's filter from outside (e.g. "focus this channel" deep-links here). Applied
+  // whenever its value changes; the user can still clear it from the header afterwards.
+  externalFilter?: { key: string; value: string } | null;
 }) {
   const { t } = useTranslation();
   const initial = loadPersist(persistKey);
@@ -93,6 +97,13 @@ export default function DataTable<T>({
     if (persistKey) localStorage.setItem(persistKey, JSON.stringify({ sort, filters, page, size }));
   }, [persistKey, sort, filters, page, size]);
 
+  useEffect(() => {
+    if (!externalFilter) return;
+    setFilters((f) => ({ ...f, [externalFilter.key]: externalFilter.value }));
+    setPage(0);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [externalFilter?.key, externalFilter?.value]);
+
   useEffect(() => {
     if (!openFilter) return;
     function onDown(e: MouseEvent) {