feat(settings): Inter as default UI font + UI font picker (T-460)

Vendor Inter (variable + italic, OFL) under assets/fonts/inter/ and make it
the default application UI face, replacing Josefin Sans (which stays bundled
as a selectable option). pubspec font family + licenses.yaml entry per D-42.

Settings → Appearance gains a UI-font select (Inter / Josefin Sans). The root
DefaultTextStyle reads app.ui.font (kUiFontSettingKey) over the default and
re-applies live on settings change, so a pick takes effect immediately. Bump
the default UI weight to w400 — Inter reads better at Regular than Josefin's
Light.

The monospace picker is deferred to T-471: clideMonoFamily is a const at ~38
call sites (not inherited), so a live mono picker needs the family routed
through context first.

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

lib/builtin/theme_picker/src/extension.dart

@@ -4,6 +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;
 
 class ThemePickerExtension extends ClideExtension {
   @override
@@ -55,6 +56,22 @@ class ThemePickerExtension extends ClideExtension {
               ),
             ],
           ),
+          SettingsSection(
+            label: 'Typography',
+            fields: [
+              SettingsField(
+                key: kUiFontSettingKey,
+                kind: SettingsFieldKind.select,
+                label: 'UI font',
+                help: 'Typeface for the app interface; applies live.',
+                defaultValue: 'Inter',
+                options: [
+                  SettingsOption(value: 'Inter', label: 'Inter'),
+                  SettingsOption(value: 'JosefinSans', label: 'Josefin Sans'),
+                ],
+              ),
+            ],
+          ),
         ],
       ),
     ),