Autosave answers

### Description
Users want to autosave answers because they don't want to lose work if a tab closes or connectivity drops.

[Working doc](https://docs.google.com/document/d/14ypEzFeVTAFszQ6LvUwB1tTzfl2qgdSbvWhtDJHgi98/edit?tab=t.di72gmc3bn34)

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### Checklist
- **Does it need UI/UX design?**
- [X] Yes. Describe the UI/UX requirements or attach a Figma link in the "Design" section.
- [ ] No
- **Does it need API design?**
- [ ] Yes. Specify the endpoints, methods, or data structures needed.
- [ ] No
- **Does it need design around state flow?**
- [ ] Yes. Outline the state flow requirements.
- [ ] No
- **Does it need a test plan?**
- [ ] Yes. Add the test plan as a comment in this task or attach the document.
- [ ] No
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### User Stories

As someone filling out a web form
I want to still see the values after I refresh or close my browser window
So that I don't lose data

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### Design

- Figma
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### Additional Notes

- Is this going to be handled by Web Forms or the host application?
- If it's handled by Web Forms, it should be opt-in
- There should be indication that values are saved and an option to reset those values
- This is the equivalent of autosave in Collect and doesn't imply any kind of draft experience

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### Related Issues

- https://github.com/getodk/web-forms/issues/573

Start with the linked working document and the Design section, then determine whether autosave belongs in Web Forms or the host application. Clarify the opt-in behavior, saved-state indication, and reset option before implementation; done means the agreed user stories and design requirements are covered.

Written by the indexing model from the issue text.