Grok Desktop Universal: macOS builds, tearable multi-window tabs, and cross-platform release CI

[perlytiara]

Summary

This PR brings together the cross-platform work from the Grok Desktop Universal fork:

• macOS support (Apple Silicon / arm64) with a native .icns icon and DMG/ZIP packaging
• Browser-style tearable tabs — drag a tab out to pop it into its own window, drag it back onto another window's tab strip to merge
• Multi-select tabs — Cmd/Ctrl-click and Shift-click to move several tabs at once
• Cross-platform release CI — a GitHub Actions workflow (.github/workflows/build.yml) that builds and publishes macOS, Windows, and Linux artifacts on v* tags

Supersedes #24 (which only contained the initial macOS build support).

Changes

• src/main.js — multi-window management, native drag-ghost window, tab drag/drop IPC, per-window targeting for close/always-on-top
• index.html — click-and-drag tab tearing, multi-select, drag shield, reorder, merge-on-drop
• package.json — macOS build config + .icns icon, pinned electron-builder@24.13.3, rebrand to grok-desktop-universal v1.3.0
• .github/workflows/build.yml — matrix build (macOS/Windows/Linux) with artifact upload and tagged GitHub Releases
• README.md / CHANGELOG.md — documented new features and platforms

Test plan

• npm run build-mac-arm64 produces a working DMG on Apple Silicon
• npm run build produces Windows installers
• Drag a tab out of the window → new window is created at the cursor
• Drag a tab back onto another window's tab strip → tabs merge
• Push a v* tag → Actions builds and attaches artifacts to a Release

Made with Cursor