feat(header): move the sync status into a chip on the header

The rail spent a two-row block plus a divider on per-user sync status. It's
global, not per-module, so it now rides the right end of the floating header as
a pill: the headline count at a glance, an accent dot when something wants you
(paused / missing history), and the counts, state, the channel-manager link and
pause in a popover.

Frees the rail's whole top block. Also drops SyncStatus's two now-dead layouts:
"bar" was a leftover of the top bar removed in 0.42.0, and "rail" lost its only
caller here — the component is chip-only, ~60 lines lighter, and the popover
reuses useDismiss.

frontend/src/App.tsx

@@ -786,12 +786,6 @@ export default function App() {
         language={i18n.language as LangCode}
         collapsed={navCollapsed}
         onToggleCollapse={() => setNavCollapsed(!navCollapsed)}
-        onGoToFullHistory={() => {
-          setChannelFilter("needs_full");
-          setChannelsView("subscribed"); // the status filter applies to the subscriptions tab
-          setChannelsFilterReset((n) => n + 1); // drop any stale column filter hiding the rows
-          setPage("channels");
-        }}
       />
       {/* Full-height filter sidebar: feed only, and hidden while a channel page overlays the
           content column. Sits beside the nav rail as its own collapsible column. */}
@@ -866,6 +860,12 @@ export default function App() {
           page={page}
           setPage={setPage}
           onYtSearch={enterYtSearch}
+          onGoToFullHistory={() => {
+            setChannelFilter("needs_full");
+            setChannelsView("subscribed"); // the status filter applies to the subscriptions tab
+            setChannelsFilterReset((n) => n + 1); // drop any stale column filter hiding the rows
+            setPage("channels");
+          }}
         />
         {/* The header floats (fixed) over the content, so it no longer occupies flow space; this
             wrapper clears it with a top pad. (All left rails, incl. Playlists, are App-level