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/ConfigPanel.field.dom.test.tsx

@@ -46,6 +46,7 @@ vi.mock("../lib/api", async (importOriginal) => {
           groups: {
             g: [text("site_name"), { ...text("signups_open"), type: "bool", value: "false" }],
           },
+          sections: [{ id: "s", groups: ["g"] }],
           secrets_manageable: true,
         }),
     },