test(e2e): add comprehensive Timeline E2E tests for EPIC-06

[steilerDev]

Summary

• Replaces the stub TimelinePage POM with a full implementation covering all EPIC-06 features (401 lines, 50+ locators and helpers)
• Adds 5 new E2E test files in e2e/tests/timeline/ with comprehensive coverage:
  • timeline-gantt.spec.ts — Gantt chart render, sidebar, zoom controls, arrow toggle, empty/no-dates states, sidebar navigation, dark mode
  • timeline-milestones.spec.ts — Milestone panel CRUD (create/edit/delete), diamond marker assertions, filter dropdown, form validation
  • timeline-calendar.spec.ts — View toggle (Gantt/Calendar), month/week grids, navigation, today button, URL param persistence, dark mode
  • timeline-schedule.spec.ts — Auto-schedule dialog (open/cancel/confirm), no-changes disabled state, error handling, accessibility
  • timeline-responsive.spec.ts — No horizontal scroll, mobile/tablet layout, keyboard navigation (ArrowDown/Enter on sidebar rows), ARIA roles/labels
• Adds createMilestoneViaApi / deleteMilestoneViaApi helpers to apiHelpers.ts
• Adds milestones, timeline, schedule API constants to testData.ts
• Removes Timeline stub test from stub-pages.spec.ts — Timeline has graduated to a full feature page

Test coverage overview

File	Test count	Key scenarios
timeline-gantt.spec.ts	17	Chart/sidebar render, zoom, arrows, empty states, dark mode
timeline-milestones.spec.ts	18	Panel CRUD, diamond markers, filter dropdown, validation
timeline-calendar.spec.ts	19	View toggle, month/week modes, navigation, URL params
timeline-schedule.spec.ts	11	Dialog lifecycle, no-changes, error state
timeline-responsive.spec.ts	16	No-scroll, keyboard nav, ARIA/roles

Test plan

• CI E2E smoke tests pass (the @smoke-tagged tests run automatically)
• Full E2E suite passes against beta after merge
• Timeline page heading loads on desktop and mobile (@responsive tag)
• Gantt chart renders with mocked data — chart, sidebar, header all visible
• Zoom level buttons toggle correctly via aria-pressed
• Arrow toggle button changes aria-pressed state
• Calendar view renders month grid with current month/year in period label
• Calendar navigation advances/retreats the period label
• Milestone panel opens, shows empty state, creates/edits/deletes milestones
• Auto-schedule dialog opens, cancel works, confirm sends PATCH requests
• All keyboard navigation tests pass (ArrowDown/Enter on sidebar rows)
• No horizontal scroll on body on any viewport

🤖 Generated with Claude Code

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🎉 This PR is included in version 1.10.0-beta.12 🎉

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