fix: use overSlotIndex in onTimedAreaDrop to fix wrong-slot drops

[jlunder00]

Problem

After PR #299 landed, drops on the DayTimeline timed area were still placed at the wrong slot. The pointer-events-none fix on TaskCards meant drops pass through to slot divs correctly, but the container's onTimedAreaDrop was still calling slotIndexFromClientY(e.clientY) — which depends on getBoundingClientRect() + scrollTop and gives wrong results when the timed area is scrolled.

Fix

onTimedAreaDrop now uses overSlotIndex.value ?? slotIndexFromClientY(e.clientY):

• overSlotIndex is set by the slot div's @dragover handler via closure-captured i — exact, no coordinate math
• The slotIndexFromClientY fallback only fires when overSlotIndex is null (drops in the hour-label gutter column, which has no slot divs)

Test Added

New flow test in DayTimelineDropZone.test.ts:

1. dragenter on timed area
2. dragover on [data-time="09:00"] slot div (sets overSlotIndex = 12)
3. drop on container with clientY: 0 (would give slot 0 via math — wrong)
4. Asserts promoteTask called with T09:00/T09:30, not T06:00

Anchor-to-Anchor

The anchor→anchor drop regression (task snaps back to source) is a separate pre-existing issue — this diff does not touch AnchorBlock.vue, useDropZone.ts, or PlanView.vue. Flagged for a separate task.

Test Results

553/553 tests pass. Build: zero type errors.