feat: desktop get locale/dialect from Tauri

[hippietrail]

Issues

Closes #3501

Description

Sets the Dialect for Harper Desktop from the system using the official Tauri method.

How Has This Been Tested?

• Manually - let me know if you can think of a way to make any unit test(s) or integration test(s)
• cargo test
• just build-desktop-macos then installing the newly build app, running it, going into prefs, and seeing this:

AI Disclosure

• I am a human and didn't use any AI.
• I used LLM features of my editor, but not an agent.
• I used an AI agent interactively.
• I am an agent or I got an agent to do the work autonomously.

Actually I used Google AI to help choose which method to use and to finesse the code to something tiny and Rusty.

Checklist

• I have performed a self-review of my own code
• I have added tests to cover my changes
• I have considered splitting this into smaller pull requests.

[hippietrail]

Building the unsigned one locally via just I got this error:

   Compiling harper-dictionary-wordlist v2.2.1 (/Users/hippietrail/harper-the-third/harper/harper-dictionary-wordlist)
    Finished `release` profile [optimized] target(s) in 7m 00s
       Built application at: /Users/hippietrail/.hippietrail/cargo-harper-consumers/universal-apple-darwin/release/harper-desktop
    Bundling harper-desktop.app (/Users/hippietrail/.hippietrail/cargo-harper-consumers/universal-apple-darwin/release/bundle/macos/harper-desktop.app)
    Bundling harper-desktop_2.2.1_universal.dmg (/Users/hippietrail/.hippietrail/cargo-harper-consumers/universal-apple-darwin/release/bundle/dmg/harper-desktop_2.2.1_universal.dmg)
     Running bundle_dmg.sh
       Error failed to bundle project error running bundle_dmg.sh: `failed to run /Users/hippietrail/.hippietrail/cargo-harper-consumers/universal-apple-darwin/release/bundle/dmg/bundle_dmg.sh`
error: Recipe `build-desktop-macos-unsigned` failed with exit code 1

Still waiting for the signed build to complete, but in the meantime the macOS CI build has succeeded so marking this as ready for review:

[elijah-potter]

This is exactly how I would have gone about this. Thank you! I just have one nit.

[elijah-potter]

This annotation is not necessary.

[hippietrail]

At first I thought you meant the comment. I didn't really get what #[must_use] does and asked an AI if this function need one too. So much for that d-:

[elijah-potter]

Great!

[elijah-potter]

Looks great! Thank you!