Function friendlyGeminiMessage

Translate a GeminiError to one user-safe English sentence.

Decision matrix:

• 504 → "took longer than expected, try smaller file"
• 429 → "rate-limited, wait a moment"
• message contains "malformed"/"parse"/"schema"/"json" → "couldn't read result"
• message contains "blocked"/"safety" → "service declined this creative"
• default → "service is having issues, try again"