feat(timeline-editor): Add time indicator, sequence flash, and taller view

Visual Enhancements:

1. Current Time Indicator (Vertical Line)
   - Faint vertical line shows current scroll position
   - Fixed position at left: 80px with gradient fade
   - Color: rgba(78, 201, 176, 0.6) (teal/cyan)
   - Gradient: fades at top/bottom for smooth appearance
   - pointer-events: none (doesn't block interaction)
   - z-index: 100 (appears above timeline elements)
   - Guides the eye during mouse wheel navigation

2. Sequence Flash Effect
   - Brief highlight when active sequence changes
   - 0.6s animation: bright glow → normal state
   - Keyframe animation sequenceFlash:
     * Start: box-shadow 20px glow, teal border
     * End: normal 10px glow, blue border
   - Triggers on lastActiveSeqIndex change
   - Auto-removes class after 600ms
   - Helps user track which sequence is now active

3. Taller Timeline View
   - timeline-container: height = calc(100vh - 280px)
   - min-height: 500px (prevents over-shrinking)
   - Uses full vertical real estate
   - Much less squished appearance
   - Better utilization of screen space
   - Accommodates more sequences in viewport

Technical Implementation:
- Added lastActiveSeqIndex global state variable
- Wheel handler detects sequence index changes
- querySelector to find sequence div and add flash class
- setTimeout removes flash class after animation
- Fixed positioning for time indicator
- Responsive height calculation for container

User Experience:
- Visual feedback during diagonal scrolling
- Clear indication of current time position
- Smooth flash draws attention to sequence changes
- More comfortable editing with taller view
- Better spatial awareness of timeline position

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