refactor(ui): fold Settings + Stats tabs into shared Tabs (R13 S2c step 6)

Tabs gains an `orientation` prop (+ optional per-tab `icon`/`testId`) so the
two hand-rolled NAV tab-sets that duplicated its active treatment now use it:
- SettingsPanel vertical rail (appearance/notifications/account) -> vertical
  Tabs with icons; settings-tab-* testIds preserved
- Stats overview/system tabs -> Tabs (rounded-full+border -> the rounded-lg
  tab canonical, per the agreed tab shape)

The underline ChannelPage tabs and the icon NavSidebar rail are intentionally
left as distinct idioms. Self-E2E'd: settings rail switches + persists with
icons, stats tabs toggle. gate green (tsc/eslint-0/vitest 136/knip/prettier).
Branch not merged.

frontend/src/components/Stats.tsx

@@ -10,6 +10,7 @@ import { LS, useAccountPersistedState } from "../lib/storage";
 import Tooltip from "./Tooltip";
 import { LoadingState, StateMessage } from "./QueryState";
 import FilterChipGroup from "./FilterChipGroup";
+import Tabs from "./Tabs";
 import { Section, SettingRow } from "./ui/form";
 import { useMe } from "../lib/useMe";
 
@@ -31,18 +32,14 @@ export default function Stats() {
     <div className="p-4 max-w-4xl mx-auto">
       {/* Only admins have a second (System) tab; everyone else just sees the Overview. */}
       {isAdmin && (
-        <div className="flex items-center gap-1 mb-4">
-          {(["overview", "system"] as StatsTab[]).map((id) => (
-            <button
-              key={id}
-              onClick={() => setTab(id)}
-              aria-pressed={tab === id}
-              className="pill px-3 py-1.5 rounded-full text-sm border"
-            >
-              {t(`stats.tabs.${id}`)}
-            </button>
-          ))}
-        </div>
+        <Tabs
+          active={tab}
+          onChange={setTab}
+          tabs={(["overview", "system"] as StatsTab[]).map((id) => ({
+            id,
+            label: t(`stats.tabs.${id}`),
+          }))}
+        />
       )}
 
       {tab === "system" && isAdmin ? <SystemStats /> : <Overview me={me} />}