feat(desktop): rebuild the task board around dense cards and direct manipulation

[IchenDEV]

Outcome

The desktop Task Board adopts the Multica board design. Cards carry more of what a
glance needs — a stable TASK-n number, a one-line description preview, the priority
flag and label, up to two label chips with +N, a session count behind a small chat
mark, an activity badge while the current Session is running/awaiting/failed, the open
PR count and the updated time. Lanes are fixed-width columns with their own vertical
scroll, a per-lane status tint, a count, and an add button that creates into that lane's
durable stage. Cards drag directly: a cross-lane drop writes the lane's stage, an in-lane
drop reorders at the resolved anchor, the target lane strengthens its tint, the hovered
card shows a 2px insertion edge, the source slot stays faintly visible, and the board
background pans horizontally. Snapshots move to v4 with a stable task number (legacy
v1–v3 boards migrate deterministically; a numbering glitch is repaired, never dropped),
and the lane projection aligns with the durable stages so In progress stays in Running
while attention still routes to Needs you. A card is highlighted only after an explicit
pick, using the list's accent selection tint. The list view, editor, inspector, and
"Move to …" menu keep working as before.

Change record

• Change: docs/sdlc/changes/2026-09-16-multica-kanban-design/intent.md

The linked change directory owns scope, authorization, acceptance, evidence, and residual risk.
Reuse it; do not copy its stage status or approval fields here.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected.
• Required checks passed; failures and skipped checks are explained in the record.
• Merge authorization: pending
• Release authorization: not requested

Drafts may contain proposals or failed verification. Ready PRs require accepted scope
and passing verification. CI checks records; it does not grant human approval.