Add timeline track and clip editing

[Chenhao-XU]

Fixes #10

This adds basic timeline track and clip creation plus direct manipulation.

Changes:

• add timeline store state for active dragging/resizing
• add actions for creating tracks and clips
• support double-click clip creation at the clicked timeline position
• support dragging clips horizontally and across tracks
• support left/right clip resizing
• keep pointer capture active during drag/resize interactions

Validation:

• esbuild parse/transpile checks for the changed timeline TS/TSX files
• git diff --check

Notes:

• I could not run a full project build in this environment because Bun and Git LFS are unavailable.