Notifications: local-only session reminders; drop the FCM-bridge framing

- Session-expiry reminders are purely on-device (AlarmManager); added an Android
  notification channel and a "Send a test reminder" button (Account > Notifications)
  to verify delivery without a real session
- Strip all FCM/bridge talk from unifiedPush.ts + README; UnifiedPush(ntfy) is now
  an optional no-op stub for hypothetical future server-initiated messages only

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

app/src/screens/NotificationsScreen.tsx

@@ -1,7 +1,8 @@
 import React, { useCallback, useState } from 'react';
-import { StyleSheet, Switch, Text, TouchableOpacity, View } from 'react-native';
+import { Alert, StyleSheet, Switch, Text, TouchableOpacity, View } from 'react-native';
 import { useFocusEffect } from '@react-navigation/native';
 import { useTheme } from '@/theme/ThemeContext';
+import { sendTestReminder } from '@/notifications/localReminders';
 import {
   DEFAULT_LEAD_MINUTES,
   LEAD_STEP,
@@ -80,9 +81,24 @@ export function NotificationsScreen() {
         </View>
       </View>
 
+      <TouchableOpacity
+        style={[styles.testBtn, { borderColor: colors.primary }]}
+        onPress={async () => {
+          const ok = await sendTestReminder(10);
+          Alert.alert(
+            ok ? 'Test scheduled' : 'Notifications blocked',
+            ok
+              ? 'A test reminder will appear in ~10 seconds (you can background the app).'
+              : 'Allow notifications for BigBrainParking in system settings.',
+          );
+        }}
+      >
+        <Text style={{ color: colors.primary, fontWeight: '700' }}>Send a test reminder</Text>
+      </TouchableOpacity>
+
       <Text style={[styles.footnote, { color: colors.subtext }]}>
-        Reminders are scheduled on-device (no Google services needed). Adjusts in{' '}
-        {LEAD_STEP}-minute steps, {MIN_LEAD_MINUTES}–{MAX_LEAD_MINUTES} min.
+        Reminders fire entirely on-device (AlarmManager) — no server, no push, no Google
+        services. Adjusts in {LEAD_STEP}-minute steps, {MIN_LEAD_MINUTES}–{MAX_LEAD_MINUTES} min.
       </Text>
     </View>
   );
@@ -108,5 +124,12 @@ const styles = StyleSheet.create({
   leadDisplay: { alignItems: 'center' },
   leadValue: { fontSize: 40, fontWeight: '800' },
   leadUnit: { fontSize: 13, marginTop: 2 },
-  footnote: { fontSize: 12, marginTop: 20, lineHeight: 18 },
+  testBtn: {
+    marginTop: 20,
+    borderWidth: 1.5,
+    borderRadius: 12,
+    padding: 14,
+    alignItems: 'center',
+  },
+  footnote: { fontSize: 12, marginTop: 16, lineHeight: 18 },
 });