Add support for opening multiple selected files from disk

[Keavon]

Finally, it's not so laborious to, one-by-one, open and re-save all the demo artwork every time we need to do that.

Also stabilizes the HashMap/HashSet serde serialization so the JSON doesn't reorder on every save.

[gemini-code-assist]

Code Review

This pull request introduces support for selecting multiple files in both the desktop and web frontends. It also implements deterministic serialization for HashMaps and HashSets by sorting their entries before serialization, which reduces unnecessary churn in saved-document diffs. A review comment suggests refactoring the file-picking logic in app.rs to eliminate code duplication and ensure consistent error logging across single and multiple file selection paths.