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/styles/components/permission-notification.css

@@ -35,7 +35,7 @@
 .permission-center-modal-backdrop {
   position: fixed;
   inset: 0;
-  background: color-mix(in srgb, var(--text-inverted) 55%, transparent);
+  background: var(--overlay-scrim);
   backdrop-filter: blur(4px);
   display: flex;
   align-items: center;
@@ -52,7 +52,7 @@
   border-radius: var(--radius-xl);
   border: 1px solid var(--border-base);
   background: var(--surface-base);
-  box-shadow: var(--panel-shadow, 0 12px 32px rgba(0, 0, 0, 0.25));
+  box-shadow: var(--panel-shadow-strong);
   overflow: hidden;
 }
 
@@ -234,4 +234,4 @@
     max-height: none;
     border-radius: 0;
   }
-}
+}