feat(build): add Linux AppImage target and release workflow

[matsuzaka-yuki]

Description

This pull request introduces official Linux build support by adding an AppImage packaging target and integrating it into the release pipeline.

Changes

• electron-builder configuration: Added a linux target in packages/desktop/electron-builder.yml configured for AppImage (x64), specifying executableName: percho and artifact pattern percho-linux-${arch}.${ext}.
• Release workflow: Added ubuntu-latest to the GitHub Actions build matrix in .github/workflows/release.yml, excluded unpacked directories from release artifacts, and included Linux in the published release release notes table.
• Documentation: Updated platform badges and download tables in README.md, README.zh.md, and CONTRIBUTING.md.

Verification

The changes have been thoroughly verified locally on a Linux x86_64 environment:

1. Build and Packaging: Successfully built packages/desktop/dist/percho-linux-x86_64.AppImage along with latest-linux.yml for updater metadata.
2. Runtime Execution: Launched the packaged AppImage and verified that:
   • The application window opens and renders correctly.
   • The embedded Pi backend engine, session management, and project trust checks initialize without errors.
   • The update checker correctly attempts update checks against the repository.
3. CI Checks:
   • npm run lint passed cleanly (checked 381 files, no errors).
   • npm run typecheck passed without errors across all packages.
   • npm run test passed all backend unit test suites.
   • npm run build succeeded.