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>

pnpm-lock.yaml

@@ -11,6 +11,9 @@ importers:
       '@tauri-apps/api':
         specifier: ^2
         version: 2.9.1
+      '@tauri-apps/plugin-clipboard-manager':
+        specifier: ^2.3.2
+        version: 2.3.2
       '@tauri-apps/plugin-dialog':
         specifier: ^2
         version: 2.6.0
@@ -735,6 +738,9 @@ packages:
     engines: {node: '>= 10'}
     hasBin: true
 
+  '@tauri-apps/plugin-clipboard-manager@2.3.2':
+    resolution: {integrity: sha512-CUlb5Hqi2oZbcZf4VUyUH53XWPPdtpw43EUpCza5HWZJwxEoDowFzNUDt1tRUXA8Uq+XPn17Ysfptip33sG4eQ==}
+
   '@tauri-apps/plugin-dialog@2.6.0':
     resolution: {integrity: sha512-q4Uq3eY87TdcYzXACiYSPhmpBA76shgmQswGkSVio4C82Sz2W4iehe9TnKYwbq7weHiL88Yw19XZm7v28+Micg==}
 
@@ -2286,6 +2292,10 @@ snapshots:
       '@tauri-apps/cli-win32-ia32-msvc': 2.9.6
       '@tauri-apps/cli-win32-x64-msvc': 2.9.6
 
+  '@tauri-apps/plugin-clipboard-manager@2.3.2':
+    dependencies:
+      '@tauri-apps/api': 2.9.1
+
   '@tauri-apps/plugin-dialog@2.6.0':
     dependencies:
       '@tauri-apps/api': 2.9.1