refactor(settings): ClideSettings — one live-preferences facade (D-101)

Introduce ClideSettings, a single widget-facing facade for the app's live
user preferences, namespaced by concern: ClideSettings.fonts.monoOf(context)
/ .fonts.uiOf(context), ClideSettings.theme.of(context),
ClideSettings.i18n.of(context). "Plumb once, use many."

Fonts are carried by a new root-provided ClideSettingsScope (resolved from the
font settings in root_shell, rebuilt on change); theme and i18n delegate to
their existing live providers (ClideTheme / the I18n service) so there's one
source of truth and their many consumers migrate incrementally rather than in
a big-bang. Reads outside a scope fall back to the bundled font defaults, so a
widget renders without a provider (isolated tests).

Migrate ~93 monospace-font call sites across 33 files from the clideMonoFamily
const to ClideSettings.fonts.monoOf(context) — pure refactor, identical family
when no override is set. 11 context-less helper sites (markdown static spans,
a few top-level/static builders) keep the const for now and are tracked in
T-472. Records D-101; updates the ui-design skill's font-family rule.

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

lib/widgets/src/clide_code_block.dart

@@ -5,6 +5,8 @@ import 'package:clide/kernel/src/theme/controller.dart';
 import 'package:clide/widgets/src/typography.dart';
 import 'package:flutter/widgets.dart';
 
+import 'clide_settings.dart';
+
 class ClideCodeBlock extends StatefulWidget {
   const ClideCodeBlock({super.key, required this.source, this.language});
 
@@ -50,7 +52,12 @@ class _ClideCodeBlockState extends State<ClideCodeBlock> {
   @override
   Widget build(BuildContext context) {
     final tokens = ClideTheme.of(context).surface;
-    final style = TextStyle(fontFamily: clideMonoFamily, fontFamilyFallback: clideMonoFamilyFallback, fontSize: clideFontMono, color: tokens.globalForeground);
+    final style = TextStyle(
+      fontFamily: ClideSettings.fonts.monoOf(context),
+      fontFamilyFallback: clideMonoFamilyFallback,
+      fontSize: clideFontMono,
+      color: tokens.globalForeground,
+    );
 
     final spans = _spans;
     TextSpan textSpan;