macOS 26: App icon does not follow the Auto/Dark icon style

### Expected
On macOS 26, when **System Settings → Appearance → Icon & widget style** is set to **Auto**, the Simplenote app icon should use its default appearance in Light Mode and its dark appearance in Dark Mode.

Selecting **Dark** explicitly should likewise display the dark icon variant.

### Observed
Simplenote continues to display the white/light icon artwork after macOS switches to Dark Mode. Other supported apps—including Simplenote on iOS—adapt their icons correctly.

The app's interface theme is not affected; this report concerns the application icon shown in the Dock, Applications folder, Launchpad, Spotlight, and similar system locations.

### Reproduced
1. Install and open Simplenote on macOS 26.
2. Open **System Settings → Appearance**.
3. Set **Icon & widget style** to **Auto**.
4. Switch the system appearance from **Light** to **Dark**.
5. Observe the Simplenote icon.
6. Quit and reopen Simplenote if necessary.

The icon remains in its light/white appearance.

Make|Model|macOS Version|App Version
-|-|-|-
Apple|Apple M5 Pro|26.6.1 (25G76)|2.21

### Technical investigation
**This technical investigation and possible implementation outline were prepared with assistance from ChatGPT by OpenAI. The proposed fix has not been built or tested against the Simplenote project and should be verified by the maintainers.**

The macOS project still uses conventional bitmap app-icon sets:

- [`AppIcon.appiconset/Contents.json`](https://github.com/Automattic/simplenote-macos/blob/trunk/Simplenote/Icons.xcassets/AppIcon.appiconset/Contents.json)
- [`AppIcon-Beta.appiconset/Contents.json`](https://github.com/Automattic/simplenote-macos/blob/trunk/Simplenote/Icons.xcassets/AppIcon-Beta.appiconset/Contents.json)

Both manifests contain one fixed PNG for each legacy macOS size and do not contain appearance-specific or layered icon representations. These resources were last updated in the 2020 [`New Big Sur AppIcons`](https://github.com/Automattic/simplenote-macos/commit/1de410b2409202627b7791808c9b26ef4b5bf863) commit.

The Xcode project continues to select `AppIcon` and `AppIcon-Beta` through `ASSETCATALOG_COMPILER_APPICON_NAME`:

- [`Simplenote.xcodeproj/project.pbxproj`](https://github.com/Automattic/simplenote-macos/blob/trunk/Simplenote.xcodeproj/project.pbxproj#L2462)
- [`Simplenote.xcodeproj/project.pbxproj`](https://github.com/Automattic/simplenote-macos/blob/trunk/Simplenote.xcodeproj/project.pbxproj#L2510)

The repository now specifies [Xcode 26.6](https://github.com/Automattic/simplenote-macos/blob/trunk/.xcode-version), so the current toolchain supports Apple's newer icon workflow.

### Possible fix

First, build and install the current `trunk` version with Xcode 26.6 and verify whether Xcode's automatically generated treatment is sufficient. The Mac App Store release predates macOS 26 and may not contain the metadata produced by the newer toolchain.

For a deliberate and fully supported result, migrate the macOS icons to Apple's Icon Composer format:

1. Create `AppIcon.icon` and `AppIcon-Beta.icon` resources using the existing Simplenote artwork separated into suitable background and foreground layers.
2. Configure **Default**, **Dark**, and **Mono** appearances for macOS. Mono should also be checked under the clear and tinted system styles.
3. Add the `.icon` files to the Xcode project and select the appropriate icon in each target/configuration.
4. Preserve the current production/Beta mapping, removing or renaming same-named legacy icon sets if they conflict.
5. Test the installed app in the Dock, Finder, Launchpad, and Spotlight with Auto, Light, Dark, Clear, and Tinted styles.
6. Confirm that the generated fallback remains correct on the project's older supported macOS versions.

Apple documents that an Icon Composer file can provide Default, Dark, and Mono appearances for both iOS and macOS:

- [Creating your app icon using Icon Composer](https://developer.apple.com/documentation/Xcode/creating-your-app-icon-using-icon-composer)
- [WWDC25: Create icons with Icon Composer](https://developer.apple.com/videos/play/wwdc2025/361/)

Build and install trunk with Xcode 26.6, then verify whether the current assets already adapt to macOS 26 appearance settings. Inspect Simplenote/Icons.xcassets/AppIcon.appiconset/Contents.json, AppIcon-Beta.appiconset/Contents.json, and the icon selections in Simplenote.xcodeproj/project.pbxproj. Done means production and Beta icons show the correct Auto, Light, Dark, Clear, and Tinted variants in Dock, Finder, Launchpad, and Spotlight while preserving older macOS fallbacks.

Written by the indexing model from the issue text.