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/Sidebar.tsx

@@ -36,6 +36,7 @@ import {
 } from "../lib/sidebarLayout";
 import { shareUrl } from "../lib/urlState";
 import { notify } from "../lib/notifications";
+import TagManager from "./TagManager";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
 const SORT_IDS = [
@@ -114,11 +115,13 @@ export default function Sidebar({
   setFilters,
   layout,
   setLayout,
+  onFocusChannel,
 }: {
   filters: FeedFilters;
   setFilters: (f: FeedFilters) => void;
   layout: SidebarLayout;
   setLayout: (l: SidebarLayout) => void;
+  onFocusChannel: (name: string) => void;
 }) {
   const { t } = useTranslation();
   const tagsQuery = useQuery({ queryKey: ["tags"], queryFn: api.tags });
@@ -172,7 +175,11 @@ export default function Sidebar({
       : t("sidebar.thisChannel"));
   const languages = tags.filter((t) => t.category === "language");
   const topics = tags.filter((t) => t.category === "topic");
+  // The user's own (non-system) tags — those assigned to channels in the Channel manager.
+  // Facets only cover language/topic, so these show their static channel_count.
+  const userTags = tags.filter((t) => !t.system);
   const [customDates, setCustomDates] = useState(false);
+  const [tagManagerOpen, setTagManagerOpen] = useState(false);
   const [editing, setEditing] = useState(false);
 
   const sensors = useSensors(
@@ -217,6 +224,7 @@ export default function Sidebar({
     date: true,
     language: languages.length > 0,
     topic: topics.length > 0,
+    tags: userTags.length > 0,
   };
 
   function toggleCollapse(id: WidgetId) {
@@ -393,6 +401,28 @@ export default function Sidebar({
           </>
         );
       }
+      case "tags":
+        return (
+          <div className="flex flex-wrap items-center gap-1.5">
+            {userTags.map((tg) => (
+              <TagChip
+                key={tg.id}
+                tag={tg}
+                count={tg.channel_count}
+                active={filters.tags.includes(tg.id)}
+                onClick={() => toggleTag(tg.id)}
+              />
+            ))}
+            <button
+              onClick={() => setTagManagerOpen(true)}
+              title={t("sidebar.manageTags")}
+              className="inline-flex items-center gap-1 text-xs px-2 py-1 rounded-full border border-dashed border-border text-muted hover:text-accent hover:border-accent transition"
+            >
+              <Pencil className="w-3 h-3" />
+              {t("sidebar.manageTags")}
+            </button>
+          </div>
+        );
     }
   }
 
@@ -496,6 +526,9 @@ export default function Sidebar({
           </div>
         </SortableContext>
       </DndContext>
+      {tagManagerOpen && (
+        <TagManager onClose={() => setTagManagerOpen(false)} onFocusChannel={onFocusChannel} />
+      )}
     </aside>
   );
 }