CI runs on Linux only; add macOS and Windows to the build matrix

[VGIL77]

.github/workflows/ci.yml builds and tests on ubuntu-latest only. The desktop release workflow already covers macOS-13, macOS-14, Windows-2022, and Ubuntu-22.04, but those platforms only get exercised at release time, not on every PR.

This isn't theoretical:

• native deps ship per-platform binaries (@napi-rs/canvas, node-llama-cpp, @reflink/reflink)
• path handling differs (the WSL 9P slowness on /mnt/d was one recent example)
• this is a desktop app; cross-platform breakage caught a week late is a worse story than caught on the PR

Proposal: expand the build job matrix to include macos-latest and windows-latest. Lint job can stay Linux-only, oxlint is platform-agnostic.

Heads up: pnpm cache on Windows runners has been flaky historically. If the matrix gets noisy we can split into separate jobs with per-OS retries instead.

[snakefood3232]

I see you're looking to expand the build job matrix to include macos-latest and windows-latest. I'd handle this by updating ci.yml to ensure we run tests across these new environments on every PR. I can have a PR ready within the next 24 hours. I've integrated cross-platform CI matrices in similar projects with complex dependencies, ensuring all builds function seamlessly.

[VGIL77]

Thanks for offering @snakefood3232. Leaving this unassigned since it's open to anyone, but a few things worth knowing if you want to take a swing:

• Windows runners need pnpm/action-setup@v4 with the explicit version input or the cache breaks; worth a fork test before opening the PR
• node-llama-cpp and @napi-rs/canvas ship per-platform binaries; if pnpm pulls the wrong arch on Windows, the failure shows up at test time rather than install
• Lint job stays Linux-only, no need to triple it
• Closes #17 in the PR description

Issue's open to anyone, first PR with green CI takes it.