ui: add auto theme switch mode based on system theme (#741)

Add system theme detection with automatic switching when OS theme
changes.

- Add ThemeMode type with "light", "dark", and "system" options
- Add system theme listener using matchMedia API
- Update theme toggle to cycle through System → Light → Dark
- Add combined sun/moon icon for system theme mode
- Migrate existing theme preferences to new format

ui-svelte/src/App.svelte

@@ -9,7 +9,7 @@
   import Playground from "./routes/Playground.svelte";
   import PlaygroundStub from "./routes/PlaygroundStub.svelte";
   import { enableAPIEvents } from "./stores/api";
-  import { initScreenWidth, isDarkMode, appTitle, connectionState } from "./stores/theme";
+  import { initScreenWidth, initSystemThemeListener, isDarkMode, appTitle, connectionState } from "./stores/theme";
   import { currentRoute } from "./stores/route";
 
   const routes = {
@@ -37,10 +37,12 @@
 
   onMount(() => {
     const cleanupScreenWidth = initScreenWidth();
+    const cleanupSystemTheme = initSystemThemeListener();
     enableAPIEvents(true);
 
     return () => {
       cleanupScreenWidth();
+      cleanupSystemTheme();
       enableAPIEvents(false);
     };
   });