web: move the section jumps to their own strip and grow the Theme menu

The six section links leave the top bar for a thin shelf under it, each a
bordered pill at the page's normal text size, so the bar itself stays a
row of actions and the jumps still read as buttons.

The Theme menu gains the workspace's colour groups — the landing palette
now takes its hue from the accent tokens — an Auto mode that follows the
OS scheme live, and a dropdown in place of the three font chips.

docker/frontend/src/theme/ThemeProvider.tsx

@@ -21,6 +21,12 @@ export function ThemeProvider({ children }: { children: ReactNode }) {
 
   useEffect(() => {
     applyTheme(mode, accent, fonts);
+    // Auto has to follow the OS while the page is open, not just at mount.
+    if (mode !== 'auto') return;
+    const media = window.matchMedia('(prefers-color-scheme: light)');
+    const onChange = () => applyTheme(mode, accent, fonts);
+    media.addEventListener('change', onChange);
+    return () => media.removeEventListener('change', onChange);
   }, [mode, accent, fonts]);
 
   const setMode = useCallback((next: ThemeMode) => setModeState(next), []);