feat(windows): add Windows x64 support

[urzeye]

Summary

• add unsigned Windows 10/11 x64 packaging for the regular WebView2 edition, including branded executables, portable asset names, and package verification
• provide a native Windows installer progress window, create Start menu and desktop shortcuts, and prevent user-visible console windows during installation and app launch
• use native Windows window chrome and the correct application icon, and restore app-level and editing keyboard shortcuts despite Electrobun's Windows accelerator-routing issue
• make host-dependent behavior portable by registering the Bash tool only when Bash is available and by describing Windows proxy detection accurately
• support importing shared HTTPS viewer links through File → Import from Clipboard when Windows URL-scheme registration is unavailable
• add a Windows CI gate that builds, verifies, installs, and launches the packaged application, plus Windows x64 release artifacts and documentation

Platform isolation

• preserve the existing macOS hiddenInset title bar and traffic-light offset
• preserve native macOS menu roles and accelerators; Windows-only shortcut handling is guarded in the renderer and Bun process
• preserve the existing macOS x64 header-padding workaround before signing; executable and installer rewriting only runs for Windows builds
• keep the Windows icon, installer wrapper, launcher wrapper, and smoke tests isolated from macOS and Linux runtime paths

Validation

• Linux CI passed: tests, lint, typecheck, desktop production renderer build, web build, and workflow YAML validation
• Windows CI passed: tests, lint, typecheck, unsigned Windows x64 package build, package verification, installation, and launch smoke test
• Windows package verification checks the required payload and update metadata and rejects unsupported GNU LongLink archive entries
• Windows smoke testing uses an isolated temporary application-data root and confirms that the installer exits successfully and the installed application remains running
• the installer is intentionally unsigned; an Authenticode/SmartScreen warning is expected

Current Windows scope

• Windows 10/11 x64
• regular edition using the system WebView2 runtime
• unsigned installer; no certificate or repository secrets are required
• no native Windows ARM64 or Performance/CEF package
• no Windows llm-space:// URL-scheme registration; shared HTTPS viewer links can be imported from the clipboard
• Windows Stable publication, Authenticode signing/timestamping, and full end-to-end updater testing remain out of scope