fix(timeline): align day-column gridlines with headers and bars

[johnpc]

What

The Timeline (Gantt) day-column gridlines drifted out of alignment with the day-number headers and the task bars — most severely on mobile, where the gridlines painted at roughly twice the density of the columns and read as unrelated to the dates above them.

Two root causes, both introduced when the gridlines were added:

1. Grid gap vs gridline pitch. .timeline__head and .timeline__row used gap: 2px between the 14 columns, but the gridlines on .timeline__rows are drawn with repeating-linear-gradient(..., 100% / 14) — which ignores the gap. Across 14 columns the 26px of accumulated gap shifted every header/bar progressively off its gridline. Fixed by dropping the inter-column gap to 0 on both grids (bars stay visually distinct via their own border-radius/padding).

2. Mobile background width mismatch. The @media (max-width: 600px) rule gave min-width: 720px to .timeline__head and .timeline__row (so the two-week axis scrolls horizontally with legible columns) but not to .timeline__rows — the element that carries the gridline background-image. So the background stayed viewport-width (~390px) while the columns were 720px, painting gridlines at ~28px pitch against ~51px columns. Fixed by adding .timeline__rows to the mobile min-width rule.

Token-only CSS change; no markup, behavior, or logic touched.

Demo (mobile, after)

Gridlines now land exactly on the day boundaries and each bar starts on its day's line.