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/diff/src/diff_view.dart

@@ -250,7 +250,7 @@ class _HunkView extends StatelessWidget {
       children: [
         Padding(
           padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 2),
-          child: ClideText(header, fontSize: clideFontMono, muted: true, fontFamily: clideMonoFamily),
+          child: ClideText(header, fontSize: clideFontMono, muted: true, fontFamily: ClideSettings.fonts.monoOf(context)),
         ),
         for (final lineObj in lines) _DiffLineRow(line: (lineObj as Map).cast<String, Object?>()),
       ],
@@ -294,7 +294,7 @@ class _DiffLineRow extends StatelessWidget {
               oldLineNo != null ? '${oldLineNo.toInt()}' : '',
               fontSize: clideFontMono,
               muted: true,
-              fontFamily: clideMonoFamily,
+              fontFamily: ClideSettings.fonts.monoOf(context),
               textAlign: TextAlign.right,
             ),
           ),
@@ -305,15 +305,22 @@ class _DiffLineRow extends StatelessWidget {
               newLineNo != null ? '${newLineNo.toInt()}' : '',
               fontSize: clideFontMono,
               muted: true,
-              fontFamily: clideMonoFamily,
+              fontFamily: ClideSettings.fonts.monoOf(context),
               textAlign: TextAlign.right,
             ),
           ),
           const SizedBox(width: 4),
-          ClideText(prefix, fontSize: clideFontMono, color: fg, fontFamily: clideMonoFamily),
+          ClideText(prefix, fontSize: clideFontMono, color: fg, fontFamily: ClideSettings.fonts.monoOf(context)),
           const SizedBox(width: 2),
           Expanded(
-            child: ClideText(text, fontSize: clideFontMono, color: fg, fontFamily: clideMonoFamily, maxLines: 1, overflow: TextOverflow.clip),
+            child: ClideText(
+              text,
+              fontSize: clideFontMono,
+              color: fg,
+              fontFamily: ClideSettings.fonts.monoOf(context),
+              maxLines: 1,
+              overflow: TextOverflow.clip,
+            ),
           ),
         ],
       ),