feat(board): canonical board definition and BoardView (T2)

Encode the board as blocks of stacked-section columns (C1) and render it as
the ASCII-styled DOM grid (C2) with hover highlight and a select emit. Layout
mirrors the physical board (Client Satisfaction above Product Architecture;
Macroplan above Defect Visualisation, both left of Feature Kanban) and boxes
use true ISO 216 proportions (1:√2, each step up scaled by √2). Both views
render from the one definition; tests pin section presence, click, hover,
layout order, and a snapshot.

tasks/todo.md

@@ -6,7 +6,7 @@ review.
 
 ## Phase 1 — Foundation & board spine
 - [x] **T1** Scaffold Nuxt 4 + tooling + Dockerised dev (M) — _deps: none_
-- [ ] **T2** Board-definition module (C1) + BoardView (C2) — F1 (M) — _deps: T1_
+- [x] **T2** Board-definition module (C1) + BoardView (C2) — F1 (M) — _deps: T1_
 - [ ] ⛳ **Checkpoint:** build + tests pass, board interactive, human review
 
 ## Phase 2 — File a defect (G1)