fix: resolve all Biome and Ruff lint errors across codebase

[Sealjay]

Context

Pre-existing lint errors across both frontend (Biome) and backend (Ruff):

Ruff (backend): ~7 E501 line-too-long errors in combat_mc_agent.py
Biome (frontend): Pre-existing issues including unused imports, !important in CSS, unused variables in e2e specs, optional chaining suggestions

Tasks

• Fix all Ruff E501 errors in backend/app/agents/combat_mc_agent.py
• Fix Biome lint errors in frontend source files
• Ensure bunx biome check . and uv run ruff check . pass cleanly