Add worktree Dock labels through the shared desktop launcher

[morgmart]

Outcome

Make macOS linked-worktree development apps distinguishable in the Dock without changing release icons or app identity. This feature was removed from #111 so that PR can focus on optional personal notification muting.

Starting point and integration

The original implementation remains available at efb9598, including scripts/dev-desktop.mjs, scripts/generate-dev-icon.swift, and dev/dev-desktop.test.mjs. It is a reference, not a merge-ready implementation.

Coordinate with #109, which owns desktop argument/port forwarding in scripts/desktop-dev.mjs. Extend one launcher after that interface is settled; do not reintroduce competing wrappers. Preserve forwarded arguments, port selection, errors, exit codes, and signal termination.

Known defects to resolve

1. Windows launch: the removed launcher executes the repository's bin/pnpm Bash proxy on every platform. Hermit is unavailable on Windows; preserve the inherited platform-compatible package-manager entry point and verify the actual subprocess boundary, not just returned argument arrays.
2. Stale branch label: the removed generator always writes src-tauri/target/dev-icons/icon.icns. A branch switch at the same commit changes icon bytes without changing the Tauri configuration. Locked tauri-codegen 2.6.3 embeds a content-hashed cached copy; the app's ordinary tauri_build::build() does not register the source icon as an input. A warm-Cargo probe using the actual cached-icon code reproduced old embedded bytes after changing the source file in place. Use a digest-keyed output/config path or another verified invalidation mechanism.

Acceptance

• Ordinary checkouts and non-macOS launches retain existing behavior; generation failures warn and fall back to the normal icon.
• Linked worktrees show the branch suffix; detached checkouts use their directory name.
• Warm build → rename/switch branch → relaunch displays the new label without cleaning Cargo output. Verify newly embedded bytes as well as the real Dock appearance.
• Exercise the real launch boundary on Windows and macOS, including forwarded arguments and termination.
• Production icons, identity, ports, and notification settings are unaffected.

A generated PNG/ICNS preview and the underlying cache probe were checked previously; no actual native Dock launch was performed. That acceptance remains required for this follow-up.