feat(desktop): add file attachments and native chat window

- Add file upload support in chat (PDF, images, txt, docs, xlsx)
  - Limited to 10MB max file size
  - Only allowed extensions for security
- Use native Windows decorations for chat window
- Remove ChatFloatingWidget (replaced by native window)
- Simplify chat event listeners (window managed by Rust)
- Fix typo "sessao" -> "sessão"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/desktop/package.json

@@ -14,6 +14,7 @@
     "@radix-ui/react-scroll-area": "^1.2.3",
     "@radix-ui/react-tabs": "^1.1.13",
     "@tauri-apps/api": "^2",
+    "@tauri-apps/plugin-dialog": "^2.4.2",
     "@tauri-apps/plugin-opener": "^2",
     "@tauri-apps/plugin-process": "^2",
     "@tauri-apps/plugin-store": "^2",