Add file drag-and-drop onto the terminal

Drop files onto a terminal pane and they're copied into the container and
their in-container paths typed into the prompt, so Claude Code can read
them for reference — mirroring the existing image-paste flow.

Backend: upload_host_file_to_terminal reads the dropped host file and
writes it under /tmp/triple-c-drops/<name> in the session's container,
returning that path. Rejects directories and unreadable paths.

Frontend: TerminalView subscribes to Tauri's webview onDragDropEvent
(OS file drops are intercepted at the webview level, so HTML5 ondrop
wouldn't expose paths). The window-wide event is guarded by the pane's
`active` flag plus a bounds hit-test so a drop only affects the terminal
it landed on; multiple files are uploaded and their paths inserted
space-separated (quoted when they contain spaces).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src-tauri/src/lib.rs

@@ -178,6 +178,7 @@ pub fn run() {
             commands::terminal_commands::terminal_resize,
             commands::terminal_commands::close_terminal_session,
             commands::terminal_commands::paste_image_to_terminal,
+            commands::terminal_commands::upload_host_file_to_terminal,
             commands::terminal_commands::start_audio_bridge,
             commands::terminal_commands::send_audio_data,
             commands::terminal_commands::stop_audio_bridge,