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.

app/pages/dashboard.vue

@@ -1,6 +1,7 @@
 <template>
   <main>
     <h1>Andon — weak point dashboard</h1>
+    <BoardView />
     <p>The red-dot defect map lands in Task 7.</p>
   </main>
 </template>