Add cross-platform application menu and release 0.3.0

[FedyaLight]

GPUI 0.2.2 stores application menus on Windows and Linux without rendering a native menu bar, leaving app settings and tools inaccessible. Add a Menu button only on Windows and Linux, in the top toolbar and empty start screen that renders the current cx.get_menus() entries and dispatches their actions through window.dispatch_action.

macOS continues to use its native menu bar without the new button. The popup preserves nested settings groups and theme checkmarks, excludes macOS system menus, Hide/Show actions and Window, scrolls within the viewport, and dismisses on outside clicks or Escape. Escape also dismisses About and timeline context menus. Prepare version 0.3.0 in workspace metadata, Cargo.lock, Linux metainfo, release notes and download links.

Validation:

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace: 370 passed, one existing ignored test
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --no-deps
• Menu projection test rejects an unfiltered negative control and passes with filtering restored.
• Shared popup renderer smoke-tested on macOS with both placements enabled: scrolling, Dark/Auto selection and refreshed checkmark, About, AI agent setup, Path Fixer, outside-click dismissal, and Escape for Menu/About. The final macOS build was checked separately to ensure no in-window Menu button appears.

Windows and Linux compilation and tests run in the PR's existing CI matrix. Publishing tag v0.3.0 after merge uses the existing release workflow for all installers and source archives.