design(board): make empty columns invite the first task with a dashed CTA

[johnpc]

Design-polish pass — the empty board of a brand-new project, i.e. exactly what a user sees the moment after creating one (the first-run moment the LLM panel kept tripping on).

A fresh project comes with three empty columns (To do / In progress / Done), and each showed only a lonely, muted "Add task" link floating in an empty box — a dead-end feel with no pull toward the primary next action.

Fix (CSS-only, tokens only)

When a column has no cards (.board-col__list:empty), the "Add task" affordance becomes a full-width, centered, dashed drop-zone CTA in the accent color — clearly inviting the first task and filling the empty column. On hover it gets an accent border + soft accent fill. Populated columns are unaffected (the list isn't empty), and it reverts to the normal inline composer when you start typing (:not(.add-card--open)).

Before → after

• Desktop (three inviting empty columns): https://files.jpc.io/d/MpF8D-empty-board-cta-desktop.png
• Mobile: https://files.jpc.io/d/g0z0E-empty-board-cta-mobile.png

Full quality gate green; presentational-only. Verified against the sandbox (AWS_PROFILE=personal).