Backgrounds and padding around the screen

[nftechie]

Context

A popular look for product videos puts the recording on a colorful background, with padding, rounded corners and a soft shadow. Today FrameComposer fills the canvas with black and fits the screen edge to edge.

Where to look

• Sources/SnapcapCore/Render/FrameComposer.swift: compose(...) starts from a black CIImage and calls fit(_:into:).
• Sources/SnapcapCore/Render/OverlayChrome.swift: renders a mask, border and shadow once with Core Graphics and caches them. Reuse this approach for the screen's rounded mask and shadow.
• Sources/SnapcapCore/Layout/OverlayLayout.swift: the single source of truth for bubble geometry. Add an equivalent ScreenLayout (screen rectangle, corner radius, shadow) that both the export and the preview use.
• Sources/SnapcapCore/Model/Project.swift: add a lenient background section, and add it to renderFingerprint's Inputs.
• Sources/Snapcap/Views/EditorView.swift: PlayerStage puts PlayerLayerView edge to edge. The preview must inset it and draw the same background.
• Sources/Snapcap/Views/Components.swift: SliderRow, plus ColorPicker with Binding(rgba:), for the controls.

Proposal

• Model: "background": { "style": "none" | "color" | "gradient", "color1": {…}, "color2": {…}, "angle": 135, "padding": 64, "cornerRadius": 12, "shadow": true }. Lengths use the 1080 reference units (OverlayLayout.unitScale). An image background can come later.
• Style none must produce exactly today's output.
• The camera bubble stays positioned relative to the whole canvas, so it can sit on the background.
• The output size doesn't change; padding shrinks the screen within the same render size.

Acceptance criteria

• A Background section in the editor with style, colors, padding, corner radius and shadow, and a live preview that matches the export.
• none renders exactly as today, so the existing ExportTests stay green.
• Older project.json files open with the background off.
• Tests cover ScreenLayout math (padding at 720p and at 4K scales identically), plus an export test that samples a background pixel and a screen pixel.