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/App.tsx

@@ -32,6 +32,7 @@ import SettingsPanel from "./components/SettingsPanel";
 import OnboardingWizard from "./components/OnboardingWizard";
 import { shouldAutoOpenOnboarding } from "./lib/onboarding";
 import Toaster from "./components/Toaster";
+import ErrorDialog from "./components/ErrorDialog";
 import About from "./components/About";
 import ReleaseNotes from "./components/ReleaseNotes";
 import VersionBanner from "./components/VersionBanner";
@@ -108,6 +109,16 @@ export default function App() {
   const [aboutOpen, setAboutOpen] = useState(false);
   const [notesOpen, setNotesOpen] = useState(false);
   const [notesHighlight, setNotesHighlight] = useState<string | undefined>(undefined);
+  // "Focus this channel in the manager": jump to the Channels page and seed its name filter
+  // so the channel is isolated. Cleared when leaving the page so it doesn't re-apply later.
+  const [focusChannelName, setFocusChannelName] = useState<string | null>(null);
+  const focusChannel = (name: string) => {
+    setFocusChannelName(name);
+    setPage("channels");
+  };
+  useEffect(() => {
+    if (page !== "channels") setFocusChannelName(null);
+  }, [page]);
 
   function openReleaseNotes(highlight?: string) {
     setNotesHighlight(highlight);
@@ -271,6 +282,7 @@ export default function App() {
               setFilters={setFilters}
               layout={sidebarLayout}
               setLayout={setSidebarLayout}
+              onFocusChannel={focusChannel}
             />
           )}
           <main className="flex-1 min-w-0 overflow-y-auto">
@@ -278,6 +290,8 @@ export default function App() {
             <Channels
               canWrite={meQuery.data!.can_write}
               isAdmin={meQuery.data!.role === "admin"}
+              focusChannelName={focusChannelName}
+              onFocusChannel={focusChannel}
               statusFilter={channelFilter}
               setStatusFilter={setChannelFilter}
               onOpenWizard={() => setWizardOpen(true)}
@@ -285,6 +299,11 @@ export default function App() {
                 setFilters({ ...filters, channelId: id, channelName: name, show: "all" });
                 setPage("feed");
               }}
+              onFilterByTag={(tagId, name) => {
+                setFilters({ ...filters, tags: [tagId], channelId: undefined, channelName: undefined });
+                setPage("feed");
+                notify({ level: "info", message: t("channels.notify.filteredByTag", { name }) });
+              }}
             />
           ) : page === "stats" && meQuery.data!.role === "admin" ? (
             <Stats />
@@ -333,6 +352,7 @@ export default function App() {
       {notesOpen && (
         <ReleaseNotes onClose={() => setNotesOpen(false)} highlight={notesHighlight} />
       )}
+      <ErrorDialog />
     </div>
   );
 }