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>

pubspec.yaml

@@ -95,6 +95,12 @@ flutter:
     - assets/queries/
 
   fonts:
+    - family: Inter
+      fonts:
+        - asset: assets/fonts/inter/Inter-VariableFont.ttf
+        - asset: assets/fonts/inter/Inter-Italic-VariableFont.ttf
+          style: italic
+
     - family: JosefinSans
       fonts:
         - asset: assets/fonts/josefin_sans/JosefinSans-VariableFont.ttf