feat: add dark theme support with CSS custom properties and Naive UI integration

Implement runtime theme switching using CSS custom properties delegated through SCSS variables, with light/dark/system modes, FOUC prevention, sidebar toggle, and settings selector. Add theme-aware video assets for sidebar and chat thinking indicator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/styles/global.scss

@@ -19,6 +19,15 @@
   }
 }
 
+// Theme transition (applied programmatically on toggle, not on load)
+html.theme-transitioning,
+html.theme-transitioning *,
+html.theme-transitioning *::before,
+html.theme-transitioning *::after {
+  transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease,
+              box-shadow 0.3s ease, fill 0.3s ease, stroke 0.3s ease !important;
+}
+
 html, body, #app {
   height: 100%;
   width: 100%;
@@ -67,7 +76,7 @@ a {
 }
 
 ::selection {
-  background: rgba($accent-primary, 0.3);
+  background: rgba(var(--accent-primary-rgb), 0.3);
 }
 
 // Shared page header