chore(docs): eslint migration + extractor updates

Migrate docs to eslint flat config (build-script console override); doc
extractor points at configs/eslint.

docs/app/composables/useTheme.ts

@@ -0,0 +1,62 @@
+export type ThemePreference = 'light' | 'dark' | 'system';
+
+const STORAGE_KEY = 'docs-theme';
+
+/**
+ * Theme controller. The actual `.dark` class is set as early as possible by the
+ * inline head script (see nuxt.config) to avoid a flash; this composable keeps a
+ * reactive preference, persists it, and re-applies the resolved theme on change.
+ */
+export function useTheme() {
+  const preference = useState<ThemePreference>('theme-preference', () => 'system');
+
+  function resolve(pref: ThemePreference): 'light' | 'dark' {
+    if (pref === 'system') {
+      return import.meta.client && globalThis.matchMedia('(prefers-color-scheme: dark)').matches
+        ? 'dark'
+        : 'light';
+    }
+    return pref;
+  }
+
+  function apply(pref: ThemePreference) {
+    if (!import.meta.client) return;
+    const resolved = resolve(pref);
+    document.documentElement.classList.toggle('dark', resolved === 'dark');
+  }
+
+  function setTheme(pref: ThemePreference) {
+    preference.value = pref;
+    if (import.meta.client) {
+      if (pref === 'system') localStorage.removeItem(STORAGE_KEY);
+      else localStorage.setItem(STORAGE_KEY, pref);
+      apply(pref);
+    }
+  }
+
+  function cycle() {
+    const order: ThemePreference[] = ['light', 'dark', 'system'];
+    const next = order[(order.indexOf(preference.value) + 1) % order.length]!;
+    setTheme(next);
+  }
+
+  // Initialise reactive preference from storage on the client.
+  if (import.meta.client) {
+    onMounted(() => {
+      const stored = localStorage.getItem(STORAGE_KEY) as ThemePreference | null;
+      preference.value = stored ?? 'system';
+
+      // Track OS changes while in `system` mode.
+      const mq = globalThis.matchMedia('(prefers-color-scheme: dark)');
+      const onChange = () => {
+        if (preference.value === 'system') apply('system');
+      };
+      mq.addEventListener('change', onChange);
+      onUnmounted(() => mq.removeEventListener('change', onChange));
+    });
+  }
+
+  const resolved = computed(() => resolve(preference.value));
+
+  return { preference, resolved, setTheme, cycle };
+}