design(board): quiet empty/ordinary due dates in the list view

[johnpc]

Design-polish pass — focused on the List view (per owner direction). The loudest problem there: every undated row showed a stark, full-contrast mm/dd/yyyy placeholder plus a calendar icon, and with 8+ of them stacked the Due column read like a data-entry spreadsheet rather than a calm Asana-style list. Ordinary future dates were also rendered at the same heavy weight as overdue ones, so nothing stood out.

Fix (CSS-only, tokens only)

• The inline due-date input's resting text is now var(--tf-text-muted) (matching the read-only date variant and Asana's quiet dates), so empty placeholders and ordinary upcoming dates recede.
• The mm/dd/yyyy calendar-picker icon is hidden at rest and revealed on row-hover/focus (extends the existing DES#13 quiet-at-rest treatment; desktop hover-capable pointers only — touch keeps it).
• Overdue / due-today dates still pop red / amber — their two-class rules outrank the muted default, so the only dates that draw the eye are the ones that matter.

Before → after

Before: every row's Due column showed a bold mm/dd/yyyy + icon; overdue and ordinary dates looked identical.
After (quiet defaults, overdue still red):

• Desktop: https://files.jpc.io/d/pfTok-list-quiet-dates-desktop.png
• Mobile: https://files.jpc.io/d/XVPNw-list-quiet-dates-mobile.png

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