refactor(gui): split records list modules

[StudentWeis]

Summary

Split the board records list implementation into focused submodules so assembly, pure metrics, masonry layout, and row rendering live behind smaller boundaries.

Linked Issue

Closes #66

Changes

• move records list assembly into records_list/mod.rs
• extract pure truncation and estimation helpers into records_list/metrics.rs
• extract masonry grid geometry into records_list/masonry.rs
• extract row rendering and state bridging into records_list/row.rs
• keep focused unit tests alongside the extracted pure logic

Testing

• ./scripts/precheck.sh passes locally
• New / updated tests cover the change