Add launch-at-login support

[martinbha]

Goal

Let users configure panes to start automatically at login on macOS and Windows.

Context

A tray-resident window manager is only useful if it is running; every comparable tool (including Rectangle via LaunchOnLogin.swift and its RectangleLauncher helper) offers launch-at-login. panes currently has no way to persist across reboots.

Scope

• macOS: register as a login item. Prefer the modern SMAppService API (macOS 13+); decide the fallback story given the current unbundled dev binary. This likely depends on the app-bundle work in Add packaging and CI #9.
• Windows: add/remove a HKCU\\Software\\Microsoft\\Windows\\CurrentVersion\\Run value or a Startup-folder shortcut.
• Add a "Launch at login" toggle to the tray menu reflecting current registration state.
• Persist the preference through the config system from Add persistent configuration loading #5 and reconcile with actual OS registration on startup.
• Handle the moved/renamed binary case gracefully (stale registration should not error the app).

Implementation Notes

• SMAppService.mainApp requires a proper bundle identifier and signed bundle to behave predictably; sequence this after Add packaging and CI #9 produces a real .app bundle.
• Rectangle needed a separate launcher helper only for pre-13 macOS; panes can require macOS 13+ and skip the helper entirely.
• Keep the registration APIs behind panes-platform traits so the tray toggle code stays shared.

Acceptance Criteria

• Toggling "Launch at login" on and rebooting (or re-logging-in) starts panes on both platforms.
• Toggling it off deregisters cleanly.
• The tray checkbox reflects the real OS registration state, not just the stored preference.
• A deleted or moved binary with a stale registration does not break startup.

Validation

• cargo fmt --all -- --check
• cargo check --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• Manual login test on macOS and Windows.