feat: auth screen with auto-redirect, sync favorites/history with backend

.agents/skills/vercel-react-native-skills/rules/react-state-fallback.md

@@ -0,0 +1,56 @@
+---
+title: Use fallback state instead of initialState
+impact: MEDIUM
+impactDescription: reactive fallbacks without syncing
+tags: state, hooks, derived-state, props, initialState
+---
+
+## Use fallback state instead of initialState
+
+Use `undefined` as initial state and nullish coalescing (`??`) to fall back to
+parent or server values. State represents user intent only—`undefined` means
+"user hasn't chosen yet." This enables reactive fallbacks that update when the
+source changes, not just on initial render.
+
+**Incorrect (syncs state, loses reactivity):**
+
+```tsx
+type Props = { fallbackEnabled: boolean }
+
+function Toggle({ fallbackEnabled }: Props) {
+  const [enabled, setEnabled] = useState(defaultEnabled)
+  // If fallbackEnabled changes, state is stale
+  // State mixes user intent with default value
+
+  return <Switch value={enabled} onValueChange={setEnabled} />
+}
+```
+
+**Correct (state is user intent, reactive fallback):**
+
+```tsx
+type Props = { fallbackEnabled: boolean }
+
+function Toggle({ fallbackEnabled }: Props) {
+  const [_enabled, setEnabled] = useState<boolean | undefined>(undefined)
+  const enabled = _enabled ?? defaultEnabled
+  // undefined = user hasn't touched it, falls back to prop
+  // If defaultEnabled changes, component reflects it
+  // Once user interacts, their choice persists
+
+  return <Switch value={enabled} onValueChange={setEnabled} />
+}
+```
+
+**With server data:**
+
+```tsx
+function ProfileForm({ data }: { data: User }) {
+  const [_theme, setTheme] = useState<string | undefined>(undefined)
+  const theme = _theme ?? data.theme
+  // Shows server value until user overrides
+  // Server refetch updates the fallback automatically
+
+  return <ThemePicker value={theme} onChange={setTheme} />
+}
+```