feat(S2): validate IMAP/SMTP hostnames against injection characters

Add validateHostname / validateOptionalHostname helpers to host_utils.dart
that reject values containing @, /, \, or control characters. Wire them
into AddAccountScreen and EditAccountScreen for all host fields.

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

lib/core/utils/host_utils.dart

@@ -2,3 +2,21 @@ bool isLocalhost(String host) {
   final h = host.trim().toLowerCase();
   return h == 'localhost' || h == '127.0.0.1' || h == '::1';
 }
+
+String? validateHostname(String? value) {
+  if (value == null || value.trim().isEmpty) return 'Required';
+  return _checkHostChars(value.trim());
+}
+
+String? validateOptionalHostname(String? value) {
+  if (value == null || value.trim().isEmpty) return null;
+  return _checkHostChars(value.trim());
+}
+
+String? _checkHostChars(String h) {
+  if (h.contains(RegExp(r'[@/\\]')) ||
+      h.codeUnits.any((c) => c < 32 || c == 127)) {
+    return 'Invalid hostname';
+  }
+  return null;
+}

lib/ui/screens/add_account_screen.dart

@@ -408,7 +408,7 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
             _field(_passwordCtrl, 'Password', obscure: true),
             const Divider(height: 32),
             Text('IMAP', style: Theme.of(context).textTheme.titleSmall),
-            _field(_imapHostCtrl, 'Host'),
+            _field(_imapHostCtrl, 'Host', validator: validateHostname),
             _field(_imapPortCtrl, 'Port', keyboardType: TextInputType.number),
             if (isLocalhost(_imapHostCtrl.text.trim()))
               SwitchListTile(
@@ -418,7 +418,7 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
               ),
             const Divider(height: 32),
             Text('SMTP', style: Theme.of(context).textTheme.titleSmall),
-            _field(_smtpHostCtrl, 'Host'),
+            _field(_smtpHostCtrl, 'Host', validator: validateHostname),
             _field(_smtpPortCtrl, 'Port', keyboardType: TextInputType.number),
             if (isLocalhost(_smtpHostCtrl.text.trim()))
               SwitchListTile(
@@ -475,6 +475,7 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
     bool obscure = false,
     bool required = true,
     TextInputType? keyboardType,
+    String? Function(String?)? validator,
   }) {
     return Padding(
       padding: const EdgeInsets.symmetric(vertical: 6),
@@ -486,9 +487,10 @@ class _AddAccountScreenState extends ConsumerState<AddAccountScreen> {
           labelText: label,
           border: const OutlineInputBorder(),
         ),
-        validator: required
+        validator: validator ??
+            (required
                 ? (v) => (v == null || v.trim().isEmpty) ? 'Required' : null
-            : null,
+                : null),
       ),
     );
   }

lib/ui/screens/edit_account_screen.dart

@@ -324,11 +324,11 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
                 'IMAP (SSL/TLS)',
                 style: Theme.of(context).textTheme.titleSmall,
               ),
-              _field(_imapHostCtrl, 'Host'),
+              _field(_imapHostCtrl, 'Host', validator: validateHostname),
               _field(_imapPortCtrl, 'Port', keyboardType: TextInputType.number),
               const Divider(height: 32),
               Text('SMTP', style: Theme.of(context).textTheme.titleSmall),
-              _field(_smtpHostCtrl, 'Host'),
+              _field(_smtpHostCtrl, 'Host', validator: validateHostname),
               _field(_smtpPortCtrl, 'Port', keyboardType: TextInputType.number),
               if (isLocalhost(_smtpHostCtrl.text.trim()))
                 SwitchListTile(
@@ -348,6 +348,7 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
                     _sieveHostCtrl,
                     'Host (leave blank to use IMAP host)',
                     required: false,
+                    validator: validateOptionalHostname,
                   ),
                   _field(
                     _sievePortCtrl,
@@ -408,6 +409,7 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
     bool obscure = false,
     bool required = true,
     TextInputType? keyboardType,
+    String? Function(String?)? validator,
   }) {
     return Padding(
       padding: const EdgeInsets.symmetric(vertical: 6),
@@ -420,9 +422,10 @@ class _EditAccountScreenState extends ConsumerState<EditAccountScreen> {
           labelText: label,
           border: const OutlineInputBorder(),
         ),
-        validator: required
+        validator: validator ??
+            (required
                 ? (v) => (v == null || v.trim().isEmpty) ? 'Required' : null
-            : null,
+                : null),
       ),
     );
   }