Add API version negotiation and compatibility

[meshackyaro]

Description

No version checking between SDK and backend API. Incompatible versions fail mysteriously.

Component

SDK

Difficulty

🟡 Medium

Tasks

• Add API version header to requests
• Check backend API version on init
• Warn on version mismatches
• Document version compatibility
• Test with multiple API versions

Acceptance Criteria

• SDK sends version information
• Version mismatches detected early
• Clear compatibility guidance

Estimated Time

2 days