fix(ui): fix case mismatch in isModelDownloaded preventing model detection

list_models returns capitalized names ("Base") but isModelDownloaded
compared with toLowerCase ("base"), so already-downloaded models were
never recognized. Use case-insensitive comparison instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/pages/SettingsPage.svelte

@@ -136,7 +136,7 @@
   }
 
   function isModelDownloaded(size) {
-    return downloadedModels.includes(size.toLowerCase());
+    return downloadedModels.some(m => m.toLowerCase() === size.toLowerCase());
   }
 
   const modelDescriptions = {