feat: add navigation page and rename scripts to English prefixes

- Add U-Claw.html navigation page with OS auto-detection and live status
- Rename scripts: start → Mac-Start/Windows-Start, menu → Mac-Menu/Windows-Menu
- Update installer scripts to reference new v2 directory structure (app/core)
- Update .gitignore to exclude build artifacts and marketing assets

.gitignore

@@ -1,5 +1,11 @@
 # U-Claw build output bundle (generated release artifact)
 U-Claw/
+U-Claw-v2/
+
+# Marketing assets
+marketing/
+marketing.zip
+social-assets/
 
 # OpenClaw source (users download separately)
 openclaw-2026.3.7/