fix(timeline-editor): Fix effect dragging offset and property panel updates

Bug Fix #1: Effect Drag Offset Calculation
- Fixed erratic jumping when clicking/dragging effects
- Problem: dragOffset calculated from element edge, not timeline origin
- Old calculation:
  dragOffset.x = e.clientX - rect.left (element-relative)
- New calculation:
  dragOffset.x = e.clientX - timelineRect.left - currentLeft (timeline-relative)
- Now correctly accounts for element's current position on timeline
- Dragging feels natural, no unexpected jumps

Bug Fix #2: Effect Positioning Relative to Sequence
- Effects now correctly positioned relative to parent sequence
- Problem: Effects were using absolute timeline positions
- Old behavior: effect.startTime = newTime (absolute)
- New behavior: effect.startTime = newTime - seq.startTime (relative)
- Algorithm:
  1. Calculate newTime from mouse position (absolute timeline)
  2. Convert to relative: relativeTime = newTime - seq.startTime
  3. Store relative time in effect
  4. Rendering uses: seq.startTime + effect.startTime (absolute)
- Sequence bounds now update correctly after effect moves

Bug Fix #3: Property Panel Updates Trigger Re-render
- Property changes now properly update timeline visual
- Added updateProperties() call after renderTimeline()
- Ensures property panel shows calculated values
- Cascade of events after property change:
  a) Update data model (sequence/effect properties)
  b) renderTimeline() → recalculates sequence bounds
  c) updateProperties() → refreshes property panel display
  d) showMessage() → user feedback
- Sequence start/end bounds recalculate correctly
- Visual updates immediately reflect property changes

Technical Details:
- startDrag: dragOffset from timeline origin, not element
- onDrag: Convert absolute position to sequence-relative time
- applyProperties: Call both renderTimeline() and updateProperties()
- Sequence visual bounds: calculated dynamically during render
  * seqVisualStart = seq.startTime + min(effect.startTime)
  * seqVisualEnd = seq.startTime + max(effect.endTime)

Result: All three operations (drag, property edit, bounds calc) work correctly

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