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/components/Header.tsx

@@ -13,6 +13,7 @@ export default function Header({
   page,
   setPage,
   onOpenSettings,
+  onGoToFullHistory,
 }: {
   me: Me;
   filters: FeedFilters;
@@ -20,6 +21,7 @@ export default function Header({
   page: Page;
   setPage: (p: Page) => void;
   onOpenSettings: () => void;
+  onGoToFullHistory: () => void;
 }) {
   async function logout() {
     await fetch("/auth/logout", { method: "POST", credentials: "include" });
@@ -36,7 +38,7 @@ export default function Header({
         Sift<span className="text-accent">lode</span>
       </button>
 
-      <SyncStatus isAdmin={me.role === "admin"} />
+      <SyncStatus isAdmin={me.role === "admin"} onGoToFullHistory={onGoToFullHistory} />
 
       {page === "feed" ? (
         <div className="flex-1 max-w-xl mx-auto relative">