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>

src-tauri/Cargo.toml

@@ -26,6 +26,7 @@ tauri-plugin-store = "2.4.2"
 tauri-plugin-notification = "2"
 tauri-plugin-os = "2"
 tauri-plugin-http = "2"
+tauri-plugin-clipboard-manager = "2"
 tempfile = "3"
 semver = "1"
 chrono = { version = "0.4.43", features = ["serde"] }