feat(alerts): live coverage preview in the schedule editor - #2435

🟢 Confidence 10/10 · safe to merge. The gap and lane logic is pinned by tests that sample the preview against onCallAt across a DST change; the only open finding is the gap-length text the author declined to change.
_{1 note · tests covered · risk medium}

Adds a live coverage preview to the on-call schedule editor (from the draft, overrides included) and reuses it on the detail page, with gap bars, a gap summary and a 30-second clock. shiftTimeline and formSection gained container-query layout. Safe to merge.

• coveragePreview draws a lane per layer, the final lane and "No one" gap bars
• The editor loads the saved schedule's overrides and layers them over the draft
• shiftTimeline gains a gap tone, weekend shading, a zone label and narrow-bar labels
• The detail page redraws the same preview from the server's shifts over 1, 2 or 4 weeks

Still open from earlier reviews

• 🔵 Note · F3 · durationText drops the minutes once a gap reaches a day · apps/web/src/page/alerts/shared/schedule-coverage.ts:213

Fixed since the last review

• ✅ F7 · Two-column breakpoint lands 4px before the timeline's @min-[30rem]

_{58a34f0 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}