Decide on Windows/Linux build support

[killerwolf]

package.json's electron-builder config already has win (nsis) and linux (AppImage) targets defined, but release.yml only builds macOS (Intel + Apple Silicon).

Decision needed

• If cross-platform is a goal: wire up windows-latest/ubuntu-latest matrix jobs in release.yml (similar structure to what watchme's workflow does), test the app actually runs there (native file dialogs, trash behavior via shell.trashItem, path handling)
• If macOS-only is the actual target audience: remove the unused win/linux build config from package.json to avoid confusion for future contributors

Either way, this should be a deliberate choice rather than leftover config nobody's using.

[killerwolf]

Still open, but the blocker moved in 2.0 (ADR 0002).

Before: package.json carried Windows (nsis) and Linux (AppImage) electron-builder config that CI never exercised. The work was mostly "turn it on and find out."

Now: that config is gone, and the question is a real one rather than a configuration switch.

• GPUI itself supports Windows and Linux, so the UI layer is not the obstacle.
• preview.rs is. Everything the app can't decode natively — PDF, Word, Excel, PowerPoint, RTF, HEIC, video poster frames — goes through QLThumbnailGenerator, which is macOS-only. thumbnail_png is already #[cfg]-gated with a non-macOS stub that returns an error, so the app would build elsewhere; it would just fall back to an icon for every one of those formats.
• Shipping that would be a worse app, not a smaller one. A Windows or Linux build needs a preview backend of its own first — poppler for PDF, something for the Office formats — which is a larger piece of work than the port itself and reintroduces exactly the per-format library sprawl that leaving Electron removed.

So the decision this issue asks for is now: is a second platform worth building and maintaining a preview stack for, given macOS gives us one for free? Worth answering explicitly rather than leaving open indefinitely.