design(task): hover-reveal empty subtask due/assignee affordances

[johnpc]

Design-polish pass — a mobile-zoom hunt on the task detail's Subtasks checklist. Every subtask row showed a full "Due date" label and a dashed "+" assignee circle even when both were empty — so a 3-item checklist read as a repetitive stack of "Due date +" boilerplate, drowning out the actual subtask titles.

Fix (CSS-only, tokens only)

Applied the app's established hover-reveal pattern (DES#36/#160): on hover-capable pointers, an empty subtask's "Due date" affordance and unassigned "+" avatar are hidden at rest and revealed when the row is hovered or focused. A resting checklist now reads as just check + title (Asana-clean). Subtasks that have a date or a real assignee always show them. Touch devices keep the affordances visible (no hover to reveal them).

Before → after

• Desktop — rest (just check + title): https://files.jpc.io/d/m5geL-subtasks-rest-desktop.png
• Desktop — hovered row (affordances revealed): https://files.jpc.io/d/vr3CP-subtasks-hover-desktop.png
• Mobile (affordances kept for touch): https://files.jpc.io/d/V4ytA-subtasks-mobile.png

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