Show release notes in-app after major updates

Nobody has claimed this yet.

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

Password managers are generally "set and forget" tools. Most people just open the app to autofill a password and immediately close it without poking around the settings. Because of this, when new features are added, users often completely miss them unless they actively hunt down the changelog on GitHub or the app store.

### Describe the solution you'd like:

It would be very helpful to have a simple screen or popup showing the release notes the first time the app is opened after an update.
To keep it from getting annoying, this should only trigger for major updates that actually include new features, skipping the minor bug fix releases. It's also important to include a "Don't show release notes again" checkbox on that screen so users who don't care about updates can easily opt out forever.

### Describe alternatives you've considered:

The current alternative is relying on users to read the update notes on F-Droid or Google Play. However, with auto-updates turned on, almost nobody actually sees those. Another alternative could be putting a little "New" badge somewhere in the settings menu, but that still requires users to go digging to figure out what changed.

### Additional context:

The app keeps getting better all the time, and it's a shame when cool additions fly under the radar simply because people didn't know they were there. Thanks again for all the hard work on the project, it really is a fantastic password manager!

The issue names no files, tests, or entry points. Start by tracing how the Android app detects launches after an update and where release notes are sourced; clarify how major updates are identified and where the opt-out choice is stored. Done means release notes appear once after qualifying updates, can be permanently dismissed, and do not appear for minor fixes.

Written by the indexing model from the issue text.