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/deeplink/src/extension.dart

@@ -83,7 +83,7 @@ class _DeepLinkConfirmDialog extends StatelessWidget {
           const SizedBox(height: 6),
           ClideText('A clide:// link from outside the app is asking to:', muted: true, fontSize: clideFontSmall),
           const SizedBox(height: 8),
-          ClideText(action.describe, fontFamily: clideMonoFamily, fontSize: clideFontSmall, color: t.globalForeground),
+          ClideText(action.describe, fontFamily: ClideSettings.fonts.monoOf(context), fontSize: clideFontSmall, color: t.globalForeground),
           const SizedBox(height: 8),
           ClideText('Only allow this if you trust where the link came from.', fontSize: clideFontMeta, color: t.statusWarning),
           const SizedBox(height: 14),