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>

pubspec.yaml

@@ -123,4 +123,10 @@ flutter:
           weight: 700
         - asset: assets/fonts/jetbrains_mono/JetBrainsMono-BoldItalic.ttf
           weight: 700
+
+    - family: FiraMono
+      fonts:
+        - asset: assets/fonts/fira_mono/FiraMono-Regular.ttf
+        - asset: assets/fonts/fira_mono/FiraMono-Bold.ttf
+          weight: 700
           style: italic