fix: doSave and newFile read .created off an unresolved Promise

[adarsh-yadav1]

While going through issue #38 I traced the bug to both call sites of _getFile().

The method is async but neither doSave() (Menu.tsx) nor newFile() (NewFile.tsx)
awaited it before accessing .created on the result. JavaScript doesn't throw when
you do that — it silently gives you undefined on a Promise object, so the wrong
timestamp gets stored with no visible error.

What I changed:

• doSave() — made async, awaits _getFile, null-guards with existingData?.created ?? new Date() for the edge case where the storage entry is missing
• newFile() — made async, awaits both _getFile and _saveFile before the editor resets, which also fixes the race condition where reset could fire before the write finished
• _getFile() in LocalStorage.ts — added try/catch around the parse, explicit Promise<File | null> return type, returns null on missing or corrupt data instead of throwing

Also added a Vitest unit test covering the three _getFile cases (missing key, corrupt JSON, valid entry).

Tested manually via ionic serve — verified created is preserved across multiple saves on the same file and modified updates correctly. tsc --noEmit passes clean.