Add an option to hide the "What's New" window on session login

**Is your feature request related to a problem? Please describe.**

The new _**"What's New"**_ window introduced in 1.38 opens at every session login, not just once after an update. I restart my macOS sessions frequently, so I have to dismiss this window every time, and there is currently no setting to prevent it from appearing.

**Describe the solution you'd like**

Either:
- a "Don't show this again" checkbox directly in the "What's New" window, or
- a toggle in Settings, similar to the existing "popup window on startup" option.

**Describe alternatives you've considered**

I inspected the app's stored preferences via the `defaults` command:

- `showGUIonStartup` is already set to `0`, so the existing startup window setting is disabled — the "What's New" window bypasses it.
- `whatsNewShownVersion` exists and holds the correct value (`1.38-95`, matching my installed version 1.38 build 95), so the "already shown" state is properly persisted, yet the window still appears at every login. Editing the stored value is therefore not a workaround.
- No other preference key (`hide*`, `show*`, ...) related to this window exists in the defaults.

I also found no related setting in the app's UI and no existing issue on this topic.

**Additional context**

AlDente 1.38 (build 95), macOS 26.5.2 (25F84), launch at login enabled. A precedent: the startup popup window got a similar opt-out setting after user feedback (#867). Thanks for the great app!

Locate the Swift implementation of the "What's New" window and the code handling the whatsNewShownVersion and showGUIonStartup preferences. Compare the existing startup opt-out behavior with the precedent in issue #867, then verify that the selected setting prevents the window from appearing at session login and behaves correctly after an update.

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