Add Notifications preferences (session-expiry reminder lead time)

- New Notifications screen (Account > Notifications): enable/disable expiry
  reminders + a 5-minute-increment stepper (5–60 min), default 15 min
- localReminders.scheduleExpiryReminder now reads the persisted lead time + enabled
  flag, so it's ready to fire once the start-session flow is wired

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/features/notifications/reminderPrefs.ts

@@ -0,0 +1,37 @@
+import AsyncStorage from '@react-native-async-storage/async-storage';
+
+/**
+ * How many minutes before a session's expiry to fire the local reminder.
+ * Configurable in Notifications preferences, in 5-minute increments.
+ */
+const KEY = 'ps_reminder_lead_minutes';
+export const DEFAULT_LEAD_MINUTES = 15;
+export const MIN_LEAD_MINUTES = 5;
+export const MAX_LEAD_MINUTES = 60;
+export const LEAD_STEP = 5;
+
+/** When true, expiry reminders are enabled at all. */
+const ENABLED_KEY = 'ps_reminder_enabled';
+
+export async function getReminderLeadMinutes(): Promise<number> {
+  const raw = await AsyncStorage.getItem(KEY);
+  const n = raw != null ? parseInt(raw, 10) : NaN;
+  return Number.isFinite(n) ? n : DEFAULT_LEAD_MINUTES;
+}
+
+export async function setReminderLeadMinutes(minutes: number): Promise<void> {
+  const clamped = Math.min(
+    MAX_LEAD_MINUTES,
+    Math.max(MIN_LEAD_MINUTES, Math.round(minutes / LEAD_STEP) * LEAD_STEP),
+  );
+  await AsyncStorage.setItem(KEY, String(clamped));
+}
+
+export async function getRemindersEnabled(): Promise<boolean> {
+  const raw = await AsyncStorage.getItem(ENABLED_KEY);
+  return raw == null ? true : raw === 'true';
+}
+
+export async function setRemindersEnabled(on: boolean): Promise<void> {
+  await AsyncStorage.setItem(ENABLED_KEY, String(on));
+}