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/Feed.tsx

@@ -185,6 +185,25 @@ export default function Feed({
     [qc]
   );
 
+  const onResetState = useCallback(
+    (id: string) => {
+      // Reset to pristine (drop the whole VideoState row, incl. resume position) — the
+      // un-watch toggle can't clear an in-progress position, this can.
+      setOverrides((o) => ({ ...o, [id]: "new" }));
+      api
+        .clearState(id)
+        .then(() => qc.invalidateQueries({ queryKey: ["feed"] }))
+        .catch(() =>
+          setOverrides((o) => {
+            const next = { ...o };
+            delete next[id];
+            return next;
+          })
+        );
+    },
+    [qc]
+  );
+
   const onChannelFilter = useCallback(
     (channelId: string, channelName: string) => {
       setFilters({ ...filters, channelId, channelName });
@@ -363,6 +382,7 @@ export default function Feed({
               onState={onState}
               onToggleSave={onToggleSave}
               onChannelFilter={onChannelFilter}
+              onResetState={onResetState}
               onOpen={openVideo}
             />
           ))}
@@ -377,6 +397,7 @@ export default function Feed({
               onState={onState}
               onToggleSave={onToggleSave}
               onChannelFilter={onChannelFilter}
+              onResetState={onResetState}
               onOpen={openVideo}
             />
           ))}