Add appscreen MCP server

[flocom]

Summary

Adds an MCP (Model Context Protocol) server under mcp-server/ that generates App Store / Play Store / web marketing screenshots server-side — a Node port of appscreen's 2D rendering pipeline (app.js) using @napi-rs/canvas, no browser required.

What it does

• Tools: list_output_sizes, list_gradient_presets, generate_screenshot, generate_batch
• Transports: stdio (default) and Streamable HTTP (--http / MCP_TRANSPORT=http)
• Faithful to app.js — reuses the same output sizes, the 25 gradient presets, and the exact background / placement / text / element / popout maths, in the app's layer order.

Parameter coverage

• Background: gradient (preset / raw CSS / explicit), solid, image (cover/contain/stretch, blur, overlay) + noise
• Screenshot: scale, position, rotation, perspective, corner radius, drop shadow, device frame
• Text: headline + subheadline with full styling, plus per-language texts maps selected by language
• elements[]: text / emoji / icon / graphic on any z-layer, with laurel / badge / star frames
• popouts[]: cropped zoom callouts of the screenshot
• layout.avoidTextOverlap: measures the text block and constrains the screenshot to the free band so the headline never overlaps the image, whatever the line count

Notes

• Self-contained: nothing outside mcp-server/ is touched; node_modules/, dist/ and local example output are gitignored.
• 3D device mockups (Three.js/WebGL) are intentionally out of scope — they can't be rendered reliably headless; the full 2D pipeline is covered.

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