feat: overlays, labels, GIF export, undo/redo, and sidebar UX

[CodeWithDennis]

Summary

Expands Pairframe composition, export, and editing UX: overlays, per-side layers, per-image labels, collapsible sidebars with dirty-only reset, animated GIF export, optional easing / hold, and undo/redo for settings.

New features

Overlay (pattern marks only)

• Overlay panel: none / dots / grid / stripes / diagonal / chevron / noise
• Painted after images, before labels
• Color, opacity, density — no solid base fill (legacy solid → none)

Per-side background & overlay

• Independent Per side toggles → A | B editors
• Enabling copies A into B; split uses the split mask; overlap uses a 50/50 divide
• Default remains one shared full-canvas layer; presets persist both modes

Per-image labels

• Image A, Image B, and optional Badge
• 9-point presets (TL…BR) + custom X/Y % on the content canvas
• Labels stay fixed when split position changes
• Legacy top/middle/bottom presets migrate automatically

Sidebar UX

• Collapsible groups with scrollable panels
• Edge-to-edge layout; collapsed tint; border under every group (including the last)
• Reset icon per group — shown only when that group differs from defaults
• Open state in localStorage (pairframe.panels.v2)

Motion export: GIF + easing + hold

• New GIF format (same transition pipeline as video)
• Formats: PNG / JPG / Video (MP4·WebM) / GIF
• Hold start / Hold end within total duration
• Easing: Linear (default) or Ease in-out (opt-in)
• Shared by preview, video export, and GIF

Undo / redo

• History for settings decisions (layout, split, labels, layers, export, …)
• Images, theme, and panel open state are not tracked
• Header Undo / Redo buttons
• Shortcuts: Cmd/Ctrl+Z, Cmd/Ctrl+Shift+Z (and Cmd/Ctrl+Y)
• Slider drags coalesce (~300ms); max 50 steps
• Applying a preset records as a single history step

Behavior / impact

• Still look is unchanged until overlay, per-side, labels, or motion settings are customized
• Background owns solid fills; overlay is marks-only
• GIF uses gifenc; large source resolutions may take longer to encode
• Overlap layouts still cannot export video/GIF (split layouts only)

Test plan

• Sidebars scroll; last group shows bottom border
• Dirty group shows reset; reset restores defaults and hides icon
• Overlay marks only; per-side A/B for background and overlay
• Labels: presets + custom X/Y; split move does not drag labels
• Video/GIF preview with holds; linear vs ease-in-out
• Export MP4/WebM and GIF
• Change settings → Undo / Redo via buttons and shortcuts
• Drag a slider → one undo step; type in an input → shortcuts ignored
• Apply preset → one undo restores prior settings
• Save/load preset with new fields; old presets still load
• Light/dark theme readability

[CodeWithDennis]