Feature: Mouse auto-track for cursor-following zoom

[amitayks]

Summary

Automatic viewport tracking that follows the mouse cursor when zoomed in, creating smooth cinematic pan effects similar to Screen Studio.

Key Features

• Mouse position recording — during screen capture, the global cursor position is captured at ~20Hz via IPC (screen.getCursorScreenPoint()) and stored as a JSON file alongside the take's recording files
• Auto-track toggle — per-section per-mode toggle (autoTrack: boolean) on each anchor keyframe. When enabled, manual backgroundPanX/Y is overridden by pan values computed from the mouse trail
• Smoothing control — configurable autoTrackSmoothing parameter using exponential moving average, letting users control how tightly the viewport follows the cursor (snappy vs cinematic)
• Editor preview — getStateAtTime() computes backgroundFocusX/Y from the mouse trail when auto-track is on, with real-time preview in the editor
• FFmpeg render — mouse trail is subsampled to keypoints (~1-2/sec), converted to focus coordinates, and fed through buildNumericExpr() to generate animated zoompan expressions in the ffmpeg filter chain
• UI controls — auto-track toggle button and smoothing scrub control, visible when a section has zoom > 1.0. Auto-disables when zoom is 1.0 (no panning needed at full size)
• Per-mode state — auto-track settings are saved independently for landscape and reel modes via MODE_SPECIFIC_PROPS

Why

When users zoom into their screen recording, the viewport is statically positioned — they manually set a pan position per section. But during recording, the most interesting content is usually where the cursor is. Auto-track makes zoomed recordings automatically follow the action.

New Capabilities

• mouse-trail-capture — cursor position recording during capture, IPC, storage
• mouse-trail-data — trail lookup, smoothing algorithm, subsampling for render
• mouse-auto-pan — auto-track mode, focus computation, editor + render integration

Related Commits

• b5407ca — feat: add mouse auto-track for cursor-following zoom in editor and render

Fixes included in PR #3