design(timeline): add day-column gridlines so bars read against dated columns

[johnpc]

Design-polish pass — the Timeline (Gantt) view. The timeline had no vertical column delineation (just a hairline under the header row), so task bars floated in undifferentiated space — you couldn't tell which day a bar actually sat on or spanned to. It read as loose colored pills rather than a dated schedule.

Fix (CSS-only, tokens only)

Added faint day-column gridlines behind the bars via a repeating-linear-gradient on .timeline__rows — a 1px token-colored (--tf-border) hairline every 1/14 of the width, matching the 14-day grid. No markup or layout change; bars keep their exact positions but now sit against readable dated columns.

The payoff is biggest on mobile, where the timeline scrolls at a wider min-width — each bar's day is now unambiguous.

Before → after

• Desktop (bars aligned to day columns): https://files.jpc.io/d/rtx22-timeline-gridlines-desktop.png
• Mobile: https://files.jpc.io/d/wL477-timeline-gridlines-mobile.png

Full quality gate green (16 timeline tests pass); presentational-only. Verified against the sandbox (AWS_PROFILE=personal).