refactor(ui): shared form primitives (Switch, Section, SettingRow, HintLabel)

components/ui/form.tsx replaces the per-panel copies:
- Switch — was duplicated in SettingsPanel (Switch) + ConfigPanel (Toggle) +
  Sidebar (inline labeled toggle).
- Section (with a card variant) — was in SettingsPanel + Stats (plain) and
  AdminUsers (glass card).
- SettingRow + HintLabel — the label+hint+control row was identical in SettingsPanel + Stats.
No visual change intended (Sidebar's toggle gains the standard knob shadow).

frontend/src/components/ConfigPanel.tsx

@@ -6,6 +6,7 @@ import { api, type ConfigItem } from "../lib/api";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
 import Tabs, { usePersistedTab } from "./Tabs";
+import { Switch } from "./ui/form";
 
 // Admin Configuration page: edit DB-overridable operational settings (registry-driven from the
 // backend — adding a key there makes it appear here automatically). Edits are drafted and
@@ -241,7 +242,7 @@ function Field({
 
       <div className="flex flex-col items-end gap-1.5 shrink-0">
         {isBool ? (
-          <Toggle checked={value === "true"} onChange={(v) => onChange(v ? "true" : "false")} />
+          <Switch checked={value === "true"} onChange={(v) => onChange(v ? "true" : "false")} />
         ) : (
           <input
             type={item.secret ? "password" : isNum ? "number" : "text"}
@@ -272,20 +273,4 @@ function Field({
       </div>
     </div>
   );
-}
-
-function Toggle({ checked, onChange }: { checked: boolean; onChange: (v: boolean) => void }) {
-  return (
-    <button
-      type="button"
-      onClick={() => onChange(!checked)}
-      className={`w-9 h-5 rounded-full transition relative shrink-0 ${checked ? "bg-accent" : "bg-border"}`}
-    >
-      <span
-        className={`absolute top-0.5 w-4 h-4 rounded-full bg-white shadow transition-all ${
-          checked ? "left-[18px]" : "left-0.5"
-        }`}
-      />
-    </button>
-  );
-}
+}