ci: add GitHub Actions workflows for multi-platform builds

- Add build-linux.yml for Ubuntu/Linux x86_64 builds
  - Installs required system dependencies (webkit2gtk, GTK3, etc.)
  - Builds Tauri application for Linux platform
  - Uploads build artifacts for distribution

- Add build-macos.yml for macOS Intel and Apple Silicon builds
  - Supports both x86_64 and aarch64 architectures
  - Handles Apple certificate import and code signing
  - Creates notarized DMG installers
  - Includes Homebrew cask generation
  - Allows skipping builds and using previous artifacts

- Add release.yml for automated releases
  - Triggers on version tags (v*)
  - Orchestrates builds across all platforms
  - Creates GitHub releases with all artifacts
  - Supports manual workflow dispatch with version input