feat(kon): replace browser clipboard with arboard

- New copy_to_clipboard Tauri command using arboard 3.6
- Replaced all 5 navigator.clipboard.writeText() calls across
  DictationPage, HistoryPage, FilesPage, and viewer with invoke()
- Native clipboard access independent of WebView permissions

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

src/lib/pages/FilesPage.svelte

@@ -117,7 +117,7 @@
   }
 
   function copyAll() {
-    if (fileTranscript) navigator.clipboard.writeText(fileTranscript);
+    if (fileTranscript) invoke("copy_to_clipboard", { text: fileTranscript }).catch(() => {});
   }
 
   function handleExport(format) {