Fix excessive disk writes (3000+/sec) from project context auth updates

The projectContext.auth comparison was using reference equality (!==) which
always evaluated to true even when auth data was unchanged. This caused
accountManager.saveToDisk() to fire on every streaming token during AI responses,
resulting in thousands of disk writes per second.

Changes:
- Use value comparison (check refresh/access tokens) instead of reference equality
- Use throttled requestSaveToDisk() instead of immediate saveToDisk()

Impact:
- Before: 3000+ writes/sec, 20-200 MB/s disk I/O, high CPU usage
- After: ~0 writes during streaming, <1 MB/s disk I/O, normal CPU

Fixes excessive SSD wear and system heat issues on macOS.