Document client URL configuration per environment and handling expired auth

[developerjamiu]

Demand

Kapa "App Setup & Usage" cluster — 14 conversations / 12 users.

Problem

• The docs show localhost, the Android emulator 10.0.2.2, and publicHost, but not how to point the Flutter client at dev / staging / production (Serverpod Cloud) URLs.
• Logout methods (signOutDevice / signOutAllDevices) are documented, but not how to detect an expired token, sign out automatically, or handle a refresh failure.

Scope

• Client setup: environment-specific client initialization (dev / staging / Cloud base URLs).
• authentication/02-basics.md: detecting token expiration and signing out on expiry.