WriteIn staging-4.3.0

feat(ci): add macOS & Linux release matrix and branch-aware staging tags

- Configure multi-platform GitHub Actions release matrix for Windows x64, macOS (Apple Silicon & Intel), and Ubuntu 22.04
- Dynamically format release tag as staging-__VERSION__ when building from the staging branch
- Update tauri.conf.json bundle targets to 'all' with macOS ad-hoc signing and Debian package dependencies
- Add platform-specific file open and folder reveal commands for macOS (open, open -R) and Linux (xdg-open)
- Adapt HeaderBar search shortcut indicator to dynamically display ⌘K on macOS and Ctrl K on Windows/Linux
- Update documentation across README and docs/ to reflect multi-platform desktop support and storage paths

Signed-off-by: Carlo Dandan <carlodandan.personal@proton.me>