feat(settings): monospace font picker + bundle Fira Mono (T-471)

Settings → Appearance gains a Monospace font select (JetBrains Mono / Fira
Mono) on the schema engine, writing app.mono.font. It applies live: root_shell
resolves the family into the ClideSettingsScope (D-101) and rebuilds on the
settings change, so the ~93 migrated mono sites — terminal, diffs, code, IDs —
re-read the new family at once.

Vendor Fira Mono (Regular + Bold, OFL) under assets/fonts/fira_mono/ with the
pubspec font family + licenses.yaml entry per D-42; JetBrains Mono stays the
default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/theme_picker/src/extension.dart

@@ -4,7 +4,7 @@ import 'package:clide/builtin/theme_picker/src/settings_view.dart';
 import 'package:clide/builtin/theme_picker/src/theme_status_item.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
-import 'package:clide/widgets/widgets.dart' show kUiFontSettingKey;
+import 'package:clide/widgets/widgets.dart' show kMonoFontSettingKey, kUiFontSettingKey;
 
 class ThemePickerExtension extends ClideExtension {
   @override
@@ -70,6 +70,17 @@ class ThemePickerExtension extends ClideExtension {
                   SettingsOption(value: 'JosefinSans', label: 'Josefin Sans'),
                 ],
               ),
+              SettingsField(
+                key: kMonoFontSettingKey,
+                kind: SettingsFieldKind.select,
+                label: 'Monospace font',
+                help: 'Terminal, diffs, code, and IDs; applies live.',
+                defaultValue: 'JetBrainsMono',
+                options: [
+                  SettingsOption(value: 'JetBrainsMono', label: 'JetBrains Mono'),
+                  SettingsOption(value: 'FiraMono', label: 'Fira Mono'),
+                ],
+              ),
             ],
           ),
         ],