feat: add timeline snap guides

[AbhinRustagi]

Pull Request Template

Description

Adds magnetic snap behavior to timeline handles. When dragging or resizing a trim, zoom, speed, annotation, or blur region, its edges snap to nearby region edges, the playhead, keyframes, and timeline bounds. A live amber guideline shows the active snap target during interaction; the drag tooltip previews the snapped values; on release, the region commits to the snapped position.

Motivation

Aligning two regions on the timeline is eyeball work right now

Type of Change

• New Feature
• Bug Fix
• Refactor / Code Cleanup
• Documentation Update
• Other (please specify)

Related Issue(s)

Closes #514 and partially #511

Screenshots / Video

Screenshot (if applicable):

![Screenshot Description](path/to/screenshot.png)

Video (if applicable):

<video src="path/to/video.mp4" controls width="600"></video>

Testing

1. Load a video into the editor.
2. Add two zoom regions. Drag one near the edge of the other → its edge should snap to a perfect match, with an amber guideline appearing during the drag.
3. Resize a zoom's left edge near the right edge of an adjacent trim → snap engages on the resized edge only; the other edge stays put.
4. Drag any region near the playhead → snaps to the current time. (Guide line auto-hides when the snap target coincides with the
   playhead, so the green playback cursor isn't doubled.)
5. Add a keyframe (default shortcut), then drag a region near it → snaps to the keyframe.
6. Drag a region toward 0:00 or the video's end → snaps to the timeline bounds.
7. Drag/resize an annotation or blur near another region's edge → snaps. Then drag two annotations onto each other → they still overlap freely (annotations/blurs are snap targets but not overlap constraints).
8. Zoom the timeline in to the millisecond level and out to the full video → snap threshold scales with zoom (max(50ms, 1% of visible range)), so it stays usable at any zoom.

Checklist

• I have performed a self-review of my code.
• I have added any necessary screenshots or videos.
• I have linked related issue(s) and updated the changelog if applicable.

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