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-tauri/src/lib.rs

@@ -68,6 +68,8 @@ pub fn run() {
             // Windows
             commands::windows::open_task_window,
             commands::windows::open_viewer_window,
+            // Clipboard
+            commands::clipboard::copy_to_clipboard,
             // LLM stubs
             commands::llm::download_llm_model,
             commands::llm::check_llm_model,