feat(ui): add system/light/dark theme toggle

Add a 3-state theme toggle in folder selection and instance tabs, and update tokens/styles so light mode has readable contrast. Sync MUI surfaces and Shiki highlighting to CSS variables to prevent stale colors when switching themes.

packages/ui/src/lib/i18n/messages/en/app.ts

@@ -29,4 +29,10 @@ export const appMessages = {
 
   "releases.uiUpdated.title": "UI updated",
   "releases.uiUpdated.message": "UI is now updated to {version}.",
+
+  "theme.mode.system": "System",
+  "theme.mode.light": "Light",
+  "theme.mode.dark": "Dark",
+  "theme.toggle.title": "Theme: {mode}",
+  "theme.toggle.ariaLabel": "Theme: {mode}",
 } as const