Add troubleshooting guide for backend connection and Android permissions

[xiaohuantech]

Why

The first failed run is the highest-risk moment for a new user. OpenGUI has several moving parts: backend, model config, APK, server URL, AccessibilityService, overlay permission, and battery settings. A troubleshooting guide would reduce abandonment and convert failures into useful issue reports.

Scope

Cover common symptoms:

• Android client cannot reach backend
• emulator uses the wrong host address
• physical phone cannot connect to the laptop backend
• AccessibilityService is disabled or killed
• overlay permission blocks task feedback
• missing model API configuration
• Redis or PostgreSQL port conflicts

Suggested files

• docs/troubleshooting.md
• README and release notes link to it

Done when

The guide maps symptoms to likely causes, checks, and fixes with short commands or screenshots where useful.

[dxcsmam]

Closing this issue as part of repository issue cleanup.