feat(design): render PNG exports on the canvas server instead of through tab capture #3218

Before submitting

• I searched open and closed issues and did not find a request for this feature.
• I reviewed this request and removed credentials, tokens, private paths, hostnames, and other sensitive data.

Problem or opportunity

Follow-up from @egdev6's review of #3153 (Gentle Design canvas, export; part of #2740).

Export as PNG in the design canvas uses the browser's tab capture (getDisplayMedia): the canvas asks to share its own tab, grabs a frame and crops the artboard locally. That keeps everything in the browser, but:

• The browser asks for permission on every export.
• It depends on the browser and the platform. Some setups, such as some Wayland sessions, never offer the tab at all, so the export can only fail with a message.
• What ends up in the PNG depends on the window: its zoom, its size, and how much of the artboard is on screen.

Proposed outcome

The canvas server renders an artboard to PNG itself, for example with a headless browser at the artboard's size from design.json, and at 2× for Export as PNG at 2×. The ⋯ menu then downloads the server's PNG with no capture prompt. The output is the same on every machine, and it doesn't depend on the window. Tab capture could stay as a fallback when the server cannot render.

Alternatives considered

• Keep tab capture only. This is what ships today: it's local and needs no extra dependency, but it prompts every time and is unavailable on some platforms.
• Render in the page with an HTML-to-canvas library. That avoids the prompt but is approximate: fonts, filters and cross-origin images often differ from the real rendering.

Additional context

• Review: feat(design-canvas): export an artboard as PNG, PDF or HTML from its ⋯ menu #3153, marked non-blocking for that PR.
• A server-side render needs a headless browser dependency, which affects install size. That trade-off is worth deciding before any implementation.