docs: update serena memories after full codebase audit

docs: update serena memories after full codebase audit

- code_style_conventions: add constant unit suffixes, BaseError import
  constraint, utility usage rules, Vue UI conventions, Python conventions,
  testing patterns (AAA, __testable__, flushMicrotasks), enum merging,
  console.* rules, fix WeakMap description accuracy
- project_overview: add auth subsystem, UI server transports, design
  patterns table, coverage thresholds, update tech stack versions
- task_completion_checklist: add ruff standalone commands for Python