Plan 8 Phase 2 - mobile switcher + per-list Done strips (Slice 1 complete)

[malinfossum]

Final Slice-1 increment: the mobile single-list switcher and per-list Done strips. Frontend-only — no backend, API, schema, or test change (the suite stays 147 green, 0 warnings).

What's in it

• Per-list Done strips (Task 7): each list gets its own collapsible ✓ Done (n) strip of its completed cards, mirroring the checklist Done strip. Retires the global Done area.
• Mobile single-list switcher (Task 8): below 768px a native <select> shows one list at a time; all columns still show at ≥768px (mobile-first, current desktop layout unchanged). The selected list is remembered per board in localStorage (validated; falls back to the first list if it's gone).
• Docs (Task 9): README status → Slice 1 complete; a dev-static-cache housekeeping note added to the backlog.

Decision to confirm

Task 7 retires the global Done area, which revises the signed-off Slice-1 "Done = both" — with per-list strips, the global grouped-by-list area would render every done card a second time. Full rationale in the spec's Mobile-layout §. Henry, this is the one call to sign off.

Verified

• dotnet test → 147 passed, 0 failed, 0 warnings (backend untouched).
• Live in-browser sweep against the running app: desktop shows all columns with the switcher hidden; mobile shows one list + the switcher; switching swaps the list, keeps focus on the select, announces "Showing , cards", and persists; reopening remembers the list; per-list strip keeps focus on its toggle (F1), measures 5.38:1 contrast (F3) and a 24×24 checkbox (F2); un-check restores the card; deleting the selected list falls back to the first (self-heals).

Please check (human acceptance)

Keyboard + screen-reader pass at phone width — switcher operable and announced, per-list strips reachable, inline renames feel right. Hard-reload / disable cache first (dev static files send no Cache-Control).

Spec: docs/2026-07-08-wend-mobile-a11y-polish-design.md
Plan: docs/plans/2026-07-08-slice1-mobile-a11y-polish.md