Add TWP Softphone Flutter app and complete mobile backend API

Backend: Add /voice/token endpoint with AccessToken + VoiceGrant for
mobile VoIP, implement unhold_call() with call leg detection, wire FCM
push notifications into call queue and webhook missed call handlers,
add data-only FCM message support for Android background wake, and add
Twilio API Key / Push Credential settings fields.

Flutter app: Full softphone with Twilio Voice SDK integration, JWT auth
with auto-refresh, SSE real-time queue updates, FCM push notifications,
Material 3 UI with dashboard, active call screen, dialpad, and call
controls (mute/speaker/hold/transfer).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mobile/lib/screens/login_screen.dart

@@ -0,0 +1,158 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+import '../providers/auth_provider.dart';
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+
+class LoginScreen extends StatefulWidget {
+  const LoginScreen({super.key});
+
+  @override
+  State<LoginScreen> createState() => _LoginScreenState();
+}
+
+class _LoginScreenState extends State<LoginScreen> {
+  final _formKey = GlobalKey<FormState>();
+  final _serverController = TextEditingController();
+  final _usernameController = TextEditingController();
+  final _passwordController = TextEditingController();
+  bool _obscurePassword = true;
+
+  @override
+  void initState() {
+    super.initState();
+    _loadSavedServer();
+  }
+
+  Future<void> _loadSavedServer() async {
+    const storage = FlutterSecureStorage();
+    final saved = await storage.read(key: 'server_url');
+    if (saved != null && mounted) {
+      _serverController.text = saved;
+    }
+  }
+
+  void _submit() {
+    if (!_formKey.currentState!.validate()) return;
+
+    var serverUrl = _serverController.text.trim();
+    if (!serverUrl.startsWith('http')) {
+      serverUrl = 'https://$serverUrl';
+    }
+
+    context.read<AuthProvider>().login(
+          serverUrl,
+          _usernameController.text.trim(),
+          _passwordController.text,
+        );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final auth = context.watch<AuthProvider>();
+
+    return Scaffold(
+      body: SafeArea(
+        child: Center(
+          child: SingleChildScrollView(
+            padding: const EdgeInsets.all(24),
+            child: Form(
+              key: _formKey,
+              child: Column(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  Icon(
+                    Icons.phone_in_talk,
+                    size: 64,
+                    color: Theme.of(context).colorScheme.primary,
+                  ),
+                  const SizedBox(height: 16),
+                  Text(
+                    'TWP Softphone',
+                    style: Theme.of(context).textTheme.headlineMedium,
+                  ),
+                  const SizedBox(height: 32),
+                  TextFormField(
+                    controller: _serverController,
+                    decoration: const InputDecoration(
+                      labelText: 'Server URL',
+                      hintText: 'https://your-site.com',
+                      prefixIcon: Icon(Icons.dns),
+                      border: OutlineInputBorder(),
+                    ),
+                    keyboardType: TextInputType.url,
+                    validator: (v) =>
+                        v == null || v.trim().isEmpty ? 'Required' : null,
+                  ),
+                  const SizedBox(height: 16),
+                  TextFormField(
+                    controller: _usernameController,
+                    decoration: const InputDecoration(
+                      labelText: 'Username',
+                      prefixIcon: Icon(Icons.person),
+                      border: OutlineInputBorder(),
+                    ),
+                    validator: (v) =>
+                        v == null || v.trim().isEmpty ? 'Required' : null,
+                  ),
+                  const SizedBox(height: 16),
+                  TextFormField(
+                    controller: _passwordController,
+                    decoration: InputDecoration(
+                      labelText: 'Password',
+                      prefixIcon: const Icon(Icons.lock),
+                      border: const OutlineInputBorder(),
+                      suffixIcon: IconButton(
+                        icon: Icon(_obscurePassword
+                            ? Icons.visibility_off
+                            : Icons.visibility),
+                        onPressed: () =>
+                            setState(() => _obscurePassword = !_obscurePassword),
+                      ),
+                    ),
+                    obscureText: _obscurePassword,
+                    validator: (v) =>
+                        v == null || v.isEmpty ? 'Required' : null,
+                  ),
+                  if (auth.error != null) ...[
+                    const SizedBox(height: 16),
+                    Text(
+                      auth.error!,
+                      style: TextStyle(
+                          color: Theme.of(context).colorScheme.error),
+                    ),
+                  ],
+                  const SizedBox(height: 24),
+                  SizedBox(
+                    width: double.infinity,
+                    height: 48,
+                    child: FilledButton(
+                      onPressed: auth.state == AuthState.authenticating
+                          ? null
+                          : _submit,
+                      child: auth.state == AuthState.authenticating
+                          ? const SizedBox(
+                              width: 24,
+                              height: 24,
+                              child: CircularProgressIndicator(
+                                  strokeWidth: 2, color: Colors.white),
+                            )
+                          : const Text('Connect'),
+                    ),
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+
+  @override
+  void dispose() {
+    _serverController.dispose();
+    _usernameController.dispose();
+    _passwordController.dispose();
+    super.dispose();
+  }
+}