feat(channels): status filter + header full-history link + stable priority

- header: per-user "N without full history" count (channels_deep_pending),
  clickable with a hint -> opens the channel manager filtered to those.
- channel manager: status filter chips (All / Needs full history / Fully synced
  / Hidden); the header link deep-links to "Needs full history".
- fix: priority up/down is now an optimistic in-place cache update (no refetch /
  re-sort), so the list no longer jumps to the top and loses your scroll position;
  the new order applies on the next page load.

frontend/src/App.tsx

@@ -21,7 +21,7 @@ import Login from "./components/Login";
 import Header from "./components/Header";
 import Sidebar from "./components/Sidebar";
 import Feed from "./components/Feed";
-import Channels from "./components/Channels";
+import Channels, { type ChannelStatusFilter } from "./components/Channels";
 import Stats from "./components/Stats";
 import SettingsPanel from "./components/SettingsPanel";
 import OnboardingWizard from "./components/OnboardingWizard";
@@ -64,6 +64,7 @@ export default function App() {
   const [page, setPageState] = useState<Page>(readPage);
   const [settingsOpen, setSettingsOpen] = useState(false);
   const [wizardOpen, setWizardOpen] = useState(false);
+  const [channelFilter, setChannelFilter] = useState<ChannelStatusFilter>("all");
 
   function setFilters(next: FeedFilters) {
     setFiltersState(next);
@@ -157,6 +158,10 @@ export default function App() {
         page={page}
         setPage={setPage}
         onOpenSettings={() => setSettingsOpen(true)}
+        onGoToFullHistory={() => {
+          setChannelFilter("needs_full");
+          setPage("channels");
+        }}
       />
       <div className="flex flex-1 min-h-0">
         {page === "feed" && (
@@ -171,6 +176,8 @@ export default function App() {
           {page === "channels" ? (
             <Channels
               canWrite={meQuery.data!.can_write}
+              statusFilter={channelFilter}
+              setStatusFilter={setChannelFilter}
               onViewChannel={(id, name) => {
                 setFilters({ ...filters, channelId: id, channelName: name, show: "all" });
                 setPage("feed");