When project ID fetching failed, an empty string was returned causing 404 errors on API requests. Now uses ANTIGRAVITY_DEFAULT_PROJECT_ID as fallback:
- isFreeTier(): Returns true when tierId is undefined (free tier by default)
- Import ANTIGRAVITY_DEFAULT_PROJECT_ID constant
- Replace empty project ID returns with fallback in all code paths:
- When loadCodeAssist returns null
- When PAID tier is detected
- When non-FREE tier without project
- When onboard/managed project ID fetch fails
Matches behavior of NoeFabris/opencode-antigravity-auth implementation.
🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)
67bcd4def4