feat: Visual feedback for invalid drag-and-drop targets

[ChuxiJ]

Problem

When dragging a clip to an incompatible track type (e.g., audio clip to MIDI track), there is no visual feedback. The drop silently fails. Users don't understand why their drag didn't work.

DragGhostInfo in src/components/timeline/useClipDrag.ts shows target track but no validity indicator.

Acceptance Criteria

• Drag to invalid target: show forbidden cursor + red-tinted ghost
• Drag to valid target: show green/blue highlight on target area
• Failed drop: clip smoothly animates back to original position
• Drag tooltip: "Drag: move | Shift+Drag: copy | Ctrl+Drag: slip"
• Tooltip appears on first drag of session (dismiss after 3 uses)

Context

Discovered during UX audit. Silent failure on drag-drop is confusing and feels broken.

Part of: Experience Polish Series