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/ex_line_overlay.dart

@@ -1,4 +1,5 @@
 import 'package:clide/kernel/kernel.dart';
+import 'package:clide/widgets/src/clide_settings.dart';
 import 'package:clide/widgets/src/clide_text.dart';
 import 'package:clide/widgets/src/typography.dart';
 import 'package:flutter/widgets.dart';
@@ -160,13 +161,13 @@ class _ExLineOverlayState extends State<ExLineOverlay> {
                       padding: const EdgeInsets.all(8),
                       child: Row(
                         children: [
-                          ClideText(':', fontFamily: clideMonoFamily, color: tokens.globalTextMuted),
+                          ClideText(':', fontFamily: ClideSettings.fonts.monoOf(context), color: tokens.globalTextMuted),
                           const SizedBox(width: 4),
                           Expanded(
                             child: EditableText(
                               controller: _input,
                               focusNode: _focus,
-                              style: TextStyle(fontFamily: clideMonoFamily, fontSize: clideFontMono, color: tokens.dropdownForeground),
+                              style: TextStyle(fontFamily: ClideSettings.fonts.monoOf(context), fontSize: clideFontMono, color: tokens.dropdownForeground),
                               cursorColor: tokens.globalFocus,
                               backgroundCursorColor: tokens.globalFocus,
                               maxLines: 1,