feat: restructure repo for collaboration + add desktop app source

- Reorganize project: clean structure (portable/ u-claw-app/ usb-scripts/ website/)
- Move docs into website/, rename uclaw-scripts to usb-scripts
- Add u-claw-app/ Electron desktop source code
- Add portable/setup.sh: one-command dev environment setup
- Add portable/SkillHub.html: skill marketplace page
- Update README: dev guide, contribution workflow, project structure
- Update .gitignore: exclude runtime binaries and build artifacts
- Expand China install guide: npm mirrors, detailed onboard wizard

u-claw-app/src/preload.js

@@ -0,0 +1,7 @@
+const { contextBridge, ipcRenderer } = require('electron');
+
+contextBridge.exposeInMainWorld('uclaw', {
+  getGatewayStatus: () => ipcRenderer.invoke('get-gateway-status'),
+  openDashboard: () => ipcRenderer.invoke('open-dashboard'),
+  openConfig: () => ipcRenderer.invoke('open-config'),
+});