feat(ui): E31 S4b — the admin pages behind one rail row, Configuration as a sectioned list, Statistics as tiles and bars

- NavRailGroup: one "Admin" rail row opening a panel of the admin pages (links, so a
  middle-click still opens a tab); the rail fits 133% zoom without a hidden scroll
- sysconfig.SECTIONS orders the config groups under four headings (every group in exactly
  one, test-held); the page shows them as Settings' side list (Tabs `group` headings);
  the lone registration group reads "Registration" under "Access"
- StatTiles / StatTile / StatBars: the Statistics overview's figures as tiles, the
  per-operation quota use as horizontal bars (a zero draws none)

frontend/src/components/ui/form.tsx

@@ -72,7 +72,7 @@ export function Section({
 
 /** A label that reveals an explanatory caption on hover (dotted underline) when `hint` is set.
  * No-op styling when there's no hint, so it's safe to use unconditionally. */
-function HintLabel({ hint, children }: { hint?: string; children: ReactNode }) {
+export function HintLabel({ hint, children }: { hint?: string; children: ReactNode }) {
   return (
     <Tooltip hint={hint ?? ""}>
       <span