feat(snap): Snap Store packaging

[Nozzit]

Summary

Adds snap/snapcraft.yaml + .github/workflows/snap.yml to bring Open CAD Studio onto the Snap Store, following the same repackage-the-release-artifact pattern already shipped for the other OpenAEC Foundation apps (open-planner-studio, open-calc-studio, open-geotechniek-studio, openaec-installer) — except this repackages the released AppImage (--appimage-extract) instead of a .deb, since that's what this project's Release workflow produces for Linux.

• Snap name: open-cad-studio.
• Triggers on completion of the Release workflow (workflow_run) — matches its release: published + workflow_dispatch triggers, so there's no risk of firing on routine pushes.
• Publishing to the Snap Store only happens once SNAPCRAFT_STORE_CREDENTIALS is set as a repo secret — until then the publish step is skipped, not failed.

⚠️ This one is meaningfully different from the other OpenAEC snaps — please read before merging

OCS uses winit + wgpu (Vulkan with a GL fallback) for its window/rendering, not GTK/WebKit. That means:

• No extensions: [gnome] (that's specific to the WebView-based Tauri apps). Instead this declares explicit opengl, wayland, x11, desktop, desktop-legacy plugs and stages the Mesa/X11/Wayland runtime libraries the binary needs (ldd on the released binary shows almost no linked libs beyond libc, which strongly suggests wgpu/winit load Vulkan/GL/X11/Wayland dynamically at runtime rather than at link time — the staged packages should cover that, but this is inferred, not confirmed against a real GPU).
• I have high confidence the packaging mechanics work (verified: extracting the actual v0.9.0 AppImage locally produces exactly the file layout the manifest expects, and snapcraft.yaml validates). I have much lower confidence in the actual GPU/windowing behavior under strict confinement — that can only really be verified by installing the built snap on a real Linux desktop with a GPU and confirming the app actually renders and opens/saves DWG/DXF files, not just that snapcraft pack succeeds in CI.

Prerequisites before this can publish anything

1. The name open-cad-studio isn't registered on snapcraft.io yet.
2. SNAPCRAFT_STORE_CREDENTIALS needs to be added as a repo secret (snapcraft export-login --snaps=open-cad-studio --channels=stable).

Test plan

• Merge, then run a workflow_dispatch test build against an existing tag (e.g. v0.9.0) — build/artifact only, no publish.
• Install the resulting snap on a real Linux desktop and actually exercise it: open a DWG/DXF file, confirm 2D/3D rendering works, not just that the window opens.
• If rendering is broken under strict confinement, this may need additional plugs/interfaces (or, in the worst case, confinement: classic, which requires manual Snap Store review on every release).
• Register the name and add credentials once the above checks out, then let the next real Release publish it.

🤖 Generated with Claude Code

[UserDevtec]

Hi @Nozzit,
Could you translate the notes and text into English instead of Dutch?

[Nozzit]

I am going on vacation tomorrow, I'll do it later so don't worry when it takes a while