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/lib/api/admin.ts

@@ -64,6 +64,8 @@ export interface ConfigItem {
 }
 export interface SystemConfigData {
   groups: Record<string, ConfigItem[]>;
+  /** The page's headings, in order, each with its groups in order; every group is in one (E31). */
+  sections: { id: string; groups: string[] }[];
   secrets_manageable: boolean;
 }