✨ Add allowed extensions to file saver dialogs

[vinceglb]

Summary

• Add defaultExtension and optional allowedExtensions to the file saver API.
• Wire the new contract through JVM, Android, iOS, macOS, Windows, Linux, and Compose launchers.
• Keep the deprecated extension overload for compatibility.
• Update docs and the sample file saver screen to reflect the new behavior.
• Improve Android save-result handling for document URIs and add regression coverage.

Testing

• :filekit-dialogs:check and :filekit-dialogs-compose:check passed.
• Sample compile checks passed for JVM, Android, JS, and WASM targets.
• Added/updated unit and host tests for file saver name normalization and Android save-path behavior.