feat: add overlap layouts and video export

[CodeWithDennis]

Summary

• New Overlap layout with Cards, Side, and Stack (keeps screenshot aspect ratio, respects PNG transparency, optional shadow)
• Video export as a wipe animation at source resolution (Auto/MP4/WebM, duration, FPS, A→B / B→A)
• In-app Play preview for the wipe before exporting
• Export formats are single-select: PNG, JPG, or Video

Key changes

• resources/js/compositor.js: overlap cards without browser chrome, alpha support, optional shadow, zero-offset fix
• resources/js/app.js: overlap state, MediaRecorder wipe export, MP4/WebM picking, preview loop, single exportFormat
• resources/views/app.blade.php: Overlap controls and Video export/preview UI

Behavior / impact

• Overlap hides Split/padding (not applicable); Radius and Swap remain available
• Video works only with Vertical / Horizontal / Diagonal
• Video ignores Scale and always uses screenshot source resolution
• Unsupported containers are disabled in the UI
• Reactive canvas updates pause during video preview/export to avoid frame races
• Presets store overlap and video settings

Test plan

• Overlap → Cards / Side / Stack with two screenshots
• Offset at 0 produces flush overlap (no forced default offset)
• Transparent PNG: no white card backing
• Shadow on/off
• Play preview then Stop; slider value restored after preview
• Video export Auto / MP4 / WebM; verify extension and playback
• A→B and B→A
• Format single-select: switch PNG/JPG/Video
• Overlap + Video: export blocked / message shown
• Save/load preset with overlap + video settings