docs: update desktop app references from Tauri to Electron

[imduchuyyy]

Issue for this PR

Closes #25967

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Updates all README files and CONTRIBUTING.md to replace Tauri references with Electron, since the desktop app now uses Electron. This includes fixing artifact filenames (darwin-aarch64 → mac-arm64, darwin-x64 → mac-x64) and updating dev commands in CONTRIBUTING.md.

How did you verify your code works?

Compared filenames against actual release assets from gh release view which show opencode-desktop-mac-arm64.dmg etc. Verified CONTRIBUTING.md commands match packages/desktop/package.json scripts.

Screenshots / recordings

N/A - documentation only change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[Brendonovich]

tyty