fix(mobile): Import legacy model options without replacing choices#16

What Changed

The mobile app now imports per-model option choices, such as reasoning effort, from the separate file that earlier builds saved them in. The import runs once. Choices already saved with drafts, and any made during the current session, win over imported values for the same model. The old file is never modified or deleted.

Why

Earlier builds kept these choices in their own file, so after an update the app forgot each model's options and fell back to defaults. A missing, unreadable or malformed legacy file is logged and skipped without blocking drafts from loading, and the import is retried on the next launch.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes