desktop: native .mov preview via AVPlayerView overlay

[RichardBray]

Sub-issue of #133. Most technically novel piece.

Why

WKWebView's HTML5

Approach

• Small native module (Zig/Obj-C, following Electrobun's native-binding pattern, e.g. https://github.com/mayfer/electrobun-macos-native-blur) that creates an AVPlayerView (AVKit) as a sibling NSView.
• Position/size synced to a placeholder element in the WKWebView layout (overlay pattern, same technique used in Tauri apps embedding native content).
• Manual z-order/hit-testing handling since this isn't a first-class Electrobun API.

Fallback considered (not chosen)

FFmpeg-generated low-res H.264 proxy preview - rejected as primary approach since it doesn't meet "avoid transcoding" and native overlay is a proven pattern.

Out of scope

• FFmpeg still runs for actual effects/export, this issue is playback preview only.