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/Sidebar.tsx

@@ -36,6 +36,7 @@ import {
 } from "../lib/sidebarLayout";
 import { shareUrl } from "../lib/urlState";
 import { notify } from "../lib/notifications";
+import { Switch } from "./ui/form";
 import TagManager from "./TagManager";
 
 // Filter ids; display labels resolved at render time via t("sidebar.sort.<id>") etc.
@@ -622,19 +623,7 @@ function Toggle({
   return (
     <label className="flex items-center justify-between py-1 cursor-pointer text-sm">
       <span>{label}</span>
-      <button
-        type="button"
-        onClick={() => onChange(!checked)}
-        className={`w-9 h-5 rounded-full transition relative ${
-          checked ? "bg-accent" : "bg-border"
-        }`}
-      >
-        <span
-          className={`absolute top-0.5 w-4 h-4 rounded-full bg-white transition-all ${
-            checked ? "left-[18px]" : "left-0.5"
-          }`}
-        />
-      </button>
+      <Switch checked={checked} onChange={onChange} />
     </label>
   );
 }