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/problems/src/problems_view.dart

@@ -113,7 +113,7 @@ class _ProblemRow extends StatelessWidget {
         children: [
           Row(
             children: [
-              ClideText(problem.source, fontSize: clideFontMono, color: tokens.statusWarning, fontFamily: clideMonoFamily),
+              ClideText(problem.source, fontSize: clideFontMono, color: tokens.statusWarning, fontFamily: ClideSettings.fonts.monoOf(context)),
               const SizedBox(width: 6),
               Expanded(child: ClideText(problem.message, color: tokens.sidebarForeground, maxLines: 2)),
             ],
@@ -121,7 +121,7 @@ class _ProblemRow extends StatelessWidget {
           if (problem.hint != null)
             Padding(
               padding: const EdgeInsets.only(left: 44, top: 2),
-              child: ClideText(problem.hint!, fontSize: clideFontMono, muted: true, fontFamily: clideMonoFamily),
+              child: ClideText(problem.hint!, fontSize: clideFontMono, muted: true, fontFamily: ClideSettings.fonts.monoOf(context)),
             ),
         ],
       ),