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/builtin/search/src/search_panel_view.dart

@@ -255,7 +255,7 @@ class _Toggle extends StatelessWidget {
           ),
           child: ClideText(
             label,
-            fontFamily: clideMonoFamily,
+            fontFamily: ClideSettings.fonts.monoOf(context),
             fontSize: clideFontCaption,
             color: active ? tokens.listItemSelectedForeground : tokens.sidebarForeground,
           ),
@@ -344,7 +344,7 @@ class _MatchRow extends StatelessWidget {
             children: [
               SizedBox(
                 width: 36,
-                child: ClideText('${match.line}', fontSize: clideFontCaption, fontFamily: clideMonoFamily, color: tokens.globalTextMuted),
+                child: ClideText('${match.line}', fontSize: clideFontCaption, fontFamily: ClideSettings.fonts.monoOf(context), color: tokens.globalTextMuted),
               ),
               Expanded(child: replacement.isEmpty ? _highlighted() : _preview()),
             ],