feat: E2E shared fixtures, API mock layer, interaction + error tests
- Create shared Playwright fixtures (authenticatedPage, mockApi, etc.)
- Create API mock layer using page.route() interception
- Refactor 14 existing test files to use shared fixtures
- Add 5 new E2E test files:
- health-score-interaction: score dimension details, tab switching
- citation-flow: search, filter, detail view, export
- competitor-interaction: add/compare/remove competitors
- error-states: API 500, offline, empty data, 401 redirect
- knowledge-interaction: create KB, upload/delete documents
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