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>

.claude/skills/ui-design/SKILL.md

@@ -36,8 +36,15 @@ These apply across every reference and every surface:
 - Never use `Material*` or `Cupertino*` widgets or color constants — clide
   is `WidgetsApp` only (D-7).
 - Use `ClideText` for themed text; never bare `Text` in production widgets.
-- Typography: `clideFontMono` for code/paths/IDs, `clideFontCaption` for
+- Typography sizes: `clideFontMono` for code/paths/IDs, `clideFontCaption` for
   status/section headers, body inherits from `DefaultTextStyle`.
+- Font *family* comes from the user-selectable facade, not a const: a
+  monospace surface uses `fontFamily: ClideSettings.fonts.monoOf(context)`
+  (and `fontFamilyFallback: clideMonoFamilyFallback`); the UI face is inherited
+  via the root `DefaultTextStyle`, or `ClideSettings.fonts.uiOf(context)` when a
+  widget must set it explicitly. `clideMonoFamily` / `clideUiFamily` are the
+  facade's defaults — don't read them directly in new widgets (D-101). Same
+  facade exposes `ClideSettings.theme.of(context)` and `.i18n.of(context)`.
 
 ## Conversation-panel cards (T-305)