fix: bump version fallback to 1.18.3 to unblock Gemini 3.1 Pro

The hardcoded ANTIGRAVITY_VERSION_FALLBACK (1.15.8) predates Gemini 3.1
Pro support on the backend. When the live version fetch fails (firewall,
WSL2 network restrictions, timeout), the stale fallback is used in
User-Agent headers. The backend then rejects requests with 'not
available on this version'.

Bump the fallback to 1.18.3 (current stable) and add 9 regression tests
covering the network-failure path.

Closes #468