Gateway down fallback (optional)

[Dodhon]

Summary

Optional fallback to file-based sessions when Gateway is unreachable.

Scope

• Add flag/env to enable fallback behavior.
• If Gateway call fails, return sessions from file path.
• Show UI warning when fallback is used.

Acceptance Criteria

• With Gateway down and fallback enabled, sessions list still renders.
• Warning banner indicates Gateway failure + fallback in use.
• With fallback disabled, UI shows error and no sessions.