Improve performance of HTML maps

- Faster to generate than PDF
- With bigger layers / AOIs, performance can take a hit
- The code Moez is using (from original AST): [https://github.com/MoezLabiadh/ast-rework/blob/main/ast_web_app/ast_mapping.py](https://github.com/MoezLabiadh/ast-rework/blob/main/ast_web_app/ast_mapping.py)

Start by reading the linked ast_web_app/ast_mapping.py implementation and identify how HTML maps are generated for larger layers or AOIs. Compare its behavior with the current repository implementation, then establish a repeatable performance check; done means HTML map generation is measurably faster for the affected larger inputs without changing the map output.

Written by the indexing model from the issue text.