feat: add native clipboard support for screenshot paste

- Add Tauri clipboard-manager plugin for native clipboard access
- Fall back to native clipboard when WebView clipboard API returns empty
- Convert RGBA image data to PNG via canvas for saving
- Allow sending messages with attachments only (no text required)
- Log attached files to output with 📎 emoji
- Update send button to enable when attachments exist

Co-Authored-By: Naomi Carrigan <commits@nhcarrigan.com>

package.json

@@ -24,6 +24,7 @@
   "license": "MIT",
   "dependencies": {
     "@tauri-apps/api": "^2",
+    "@tauri-apps/plugin-clipboard-manager": "^2.3.2",
     "@tauri-apps/plugin-dialog": "^2",
     "@tauri-apps/plugin-notification": "^2",
     "@tauri-apps/plugin-opener": "^2",