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/git/src/git_status_item.dart

@@ -164,7 +164,12 @@ class _BranchPickerState extends State<_BranchPicker> {
               child: Row(
                 children: [
                   Expanded(
-                    child: ClideText('Switch branch', fontSize: clideFontCaption, color: tokens.globalTextMuted, fontFamily: clideMonoFamily),
+                    child: ClideText(
+                      'Switch branch',
+                      fontSize: clideFontCaption,
+                      color: tokens.globalTextMuted,
+                      fontFamily: ClideSettings.fonts.monoOf(context),
+                    ),
                   ),
                   GestureDetector(
                     onTap: () => widget.onDismiss(),
@@ -229,7 +234,7 @@ class _BranchRow extends StatelessWidget {
             Expanded(
               child: ClideText(
                 name,
-                fontFamily: clideMonoFamily,
+                fontFamily: ClideSettings.fonts.monoOf(context),
                 fontSize: clideFontMono,
                 color: current ? tokens.globalForeground : tokens.listItemForeground,
               ),