[M8-T04] Write file operation E2E tests

## Task: M8-T04

**Milestone:** M8 - Playwright Tests (1 day)
**Feature:** RuleZ UI

## Description

Write E2E tests for file operations.

## Acceptance Criteria

- [ ] Test file sidebar display
- [ ] Test tab management
- [ ] Test save confirmation

## Dependencies

- Depends on: M8-T01, M4-T03, M4-T04