fix: DayTimeline DnD drop always lands at slot 0 (z-order race)

[jlunder00]

Problem

Dragging a task from the plan's anchor list and dropping it onto the DayTimeline always scheduled it at 6:00 AM (slot 0) instead of the visually targeted time slot.

Root cause — z-order race:

1. Slot divs are rendered before TaskCard blocks in DOM order; TaskCards have higher z-order by default.
2. When overSlotIndex was computed from clientY (via slotIndexFromClientY), any inaccuracy in getBoundingClientRect() at drag-start (e.g., during the first dragover into the component) could produce slot 0.
3. The highlighted slot 0 got z-30 (above TaskCards). Subsequent drop events hit slot 0 regardless of the cursor position, calling onSlotDrop(e, 0) → always 6:00 AM.

Fix

Set dragActive = ref(false) in DayTimeline. On @dragenter it flips true; on @dragleave (container exit), @drop (container fallback), and @drop.stop (slot-direct) it flips false.

While dragActive is true, TaskCard blocks receive pointer-events-none. Drops over event-block areas pass through directly to the underlying slot divs, calling onSlotDrop(e, i) with the exact slot index — no slotIndexFromClientY math involved, no z-order race.

slotIndexFromClientY and onTimedAreaDrop remain as a safety net for drops landing in the hour-labels gutter column (left strip with no slot divs).

Tests

• Regression: onTimedAreaDrop maps clientY to the correct 15-min slot using rect.top + scrollTop — mocks getBoundingClientRect and verifies clientY=190 / top=100 maps to slot 4 (07:00). Passes before and after (math was always correct).
• Structural (TDD red→green): TaskCard event blocks receive pointer-events-none class while a drag is active — fails before fix, passes after.

All 539 tests pass. Build is clean (zero vue-tsc errors).

Note on approach

Team-lead's post-compact reminder suggested await nextTick() in onTimedAreaDrop. This is incorrect — e.clientY is immutable at event-fire time and getBoundingClientRect() is synchronous; there is nothing to settle with a tick. The structural pointer-events-none fix eliminates the z-order race entirely rather than patching the coordinate math.