auth: replace password login with OIDC PKCE flow (Phase 5)

- Install expo-auth-session + expo-web-browser
- Add 'bincio-rec' URL scheme to app.json for deep-link redirect
- auth.ts: generate PKCE verifier/challenge, open bincio.org/oauth2/authorize
  in browser, exchange auth code for RS256 id_token, store in AsyncStorage
- SettingsScreen: remove handle/password fields, single 'Sign in with bincio'
  button that opens the browser flow

src/screens/SettingsScreen.tsx

@@ -1,6 +1,6 @@
 import React, { useEffect, useState } from 'react';
 import {
-  View, Text, TextInput, StyleSheet, Switch,
+  View, Text, StyleSheet, Switch,
   TouchableOpacity, Alert, ScrollView, ActivityIndicator,
 } from 'react-native';
 import AsyncStorage from '@react-native-async-storage/async-storage';
@@ -131,34 +131,28 @@ function AppTab() {
 
 function SyncTab() {
   const { accent } = useTheme();
-  const [instanceUrl, setInstanceUrl] = useState('');
-  const [handle, setHandle] = useState('');
-  const [password, setPassword] = useState('');
   const [connectedAs, setConnectedAs] = useState<string | null>(null);
   const [connecting, setConnecting] = useState(false);
 
   useEffect(() => {
     loadAuthState().then((auth) => {
-      if (auth) { setInstanceUrl(auth.instanceUrl); setHandle(auth.handle); setConnectedAs(auth.handle); }
+      if (auth) setConnectedAs(auth.handle);
     });
   }, []);
 
   async function handleConnect() {
-    if (!instanceUrl.trim()) { Alert.alert('Required', 'Enter the instance URL.'); return; }
-    if (!handle.trim())      { Alert.alert('Required', 'Enter your handle.'); return; }
-    if (!password)           { Alert.alert('Required', 'Enter your password.'); return; }
     setConnecting(true);
-    const result = await login(instanceUrl.trim(), handle.trim(), password);
+    const result = await login();
     setConnecting(false);
-    if (result.ok) { setConnectedAs(result.displayName || handle.trim()); setPassword(''); }
-    else Alert.alert('Login failed', result.error ?? 'Unknown error');
+    if (result.ok) setConnectedAs(result.displayName ?? '');
+    else if (result.error !== 'Cancelled') Alert.alert('Sign in failed', result.error ?? 'Unknown error');
   }
 
   async function handleDisconnect() {
     Alert.alert('Disconnect', 'Remove saved credentials?', [
       { text: 'Cancel', style: 'cancel' },
       { text: 'Disconnect', style: 'destructive', onPress: async () => {
-        await logout(); setConnectedAs(null); setHandle(''); setPassword('');
+        await logout(); setConnectedAs(null);
       }},
     ]);
   }
@@ -167,18 +161,6 @@ function SyncTab() {
     <ScrollView contentContainerStyle={styles.content}>
       <Text style={styles.sectionTitle}>bincio instance</Text>
 
-      <Text style={styles.label}>Instance URL</Text>
-      <TextInput
-        style={styles.input}
-        value={instanceUrl}
-        onChangeText={setInstanceUrl}
-        placeholder="https://bincio.example.com"
-        placeholderTextColor={colors.placeholder}
-        autoCapitalize="none"
-        keyboardType="url"
-        editable={!connectedAs}
-      />
-
       {connectedAs ? (
         <View style={styles.connectedBox}>
           <View>
@@ -191,16 +173,9 @@ function SyncTab() {
         </View>
       ) : (
         <>
-          <Text style={styles.label}>Handle</Text>
-          <TextInput style={styles.input} value={handle} onChangeText={setHandle} placeholder="your-handle" placeholderTextColor={colors.placeholder} autoCapitalize="none" autoCorrect={false} />
-
-          <Text style={styles.label}>Password</Text>
-          <TextInput style={styles.input} value={password} onChangeText={setPassword} placeholder="••••••••" placeholderTextColor={colors.placeholder} secureTextEntry />
-
-          <Text style={styles.hint}>Your password is used once to obtain a session token, then forgotten.</Text>
-
+          <Text style={styles.hint}>Sign in with your bincio account to sync recordings.</Text>
           <TouchableOpacity style={[styles.connectBtn, connecting && styles.connectBtnDisabled]} onPress={handleConnect} disabled={connecting}>
-            {connecting ? <ActivityIndicator color={colors.text} /> : <Text style={styles.connectBtnText}>Connect</Text>}
+            {connecting ? <ActivityIndicator color={colors.text} /> : <Text style={styles.connectBtnText}>Sign in with bincio</Text>}
           </TouchableOpacity>
         </>
       )}