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/Cargo.toml

@@ -33,3 +33,4 @@ serde_json = "1"
 
 # Async runtime (spawn_blocking for inference)
 tokio = { version = "1", features = ["rt", "sync"] }
+arboard = "3.6.1"