E2E test coverage for Timeline scale toggle, Year view, allocation editing, and Gantt UX (PR #231)

[NickMonrad]

Context

PR #231 shipped a large set of Timeline enhancements that currently have no Playwright coverage:

Tests to add

gantt.spec.ts

• Scale toggle switches between Wk / Mo / Qtr / Yr and Gantt re-renders
• Year scale shows H1/H2 header labels in the SVG
• Expand All / Collapse All toggle collapses/expands epic rows and persists across timeline refetch (regression guard for the expand-state reset bug)

timeline.spec.ts

• Allocation mode dropdown in Resource Counts changes from T&M to Full Project / Timeline
• Allocation % input updates and persists
• Timeline start/end week inputs appear when mode = Timeline
• Named resource histogram shows per-person bars (not full pool) — visual sanity check via bar count

backlog.spec.ts

• CSV import with TemplateSize=S on a Story row auto-expands template tasks using Small tier hours
• CSV export includes TemplateSize column header

Notes

• All tests should use the setupTimeline() helper pattern from gantt.spec.ts for Timeline tests
• The expand/collapse regression test is particularly important — it guards against the knownEpicIds ref bug