chore: restrict LLM model list strictly to 100% Free-Tier Flash models

fix: update active model list to gemini-3.6-flash, 3.5-flash, 3.1-flash-lite, flash-latest

fix: add gemini-2.0-flash model with 1.5-flash fallback and SSE streaming support

fix: update Gemini API model to gemini-1.5-flash and improve error messaging

features- Enhanced Map and what if simlulation and heat pockets.

Initial commit of ISRO Climate Twin