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/DictationPage.svelte

@@ -332,7 +332,7 @@
   async function finaliseTranscription() {
     if (transcript.trim()) {
       if (settings.autoCopy) {
-        navigator.clipboard.writeText(transcript).catch(() => {});
+        invoke("copy_to_clipboard", { text: transcript }).catch(() => {});
       }
 
       // Save audio if enabled — capture path for history replay
@@ -395,7 +395,7 @@
   }
 
   function copyAll() {
-    if (transcript) navigator.clipboard.writeText(transcript).catch(() => {});
+    if (transcript) invoke("copy_to_clipboard", { text: transcript }).catch(() => {});
   }
 
   function clearTranscript() {