feat: use localstorage to hold on to prompt draft in web

What does this MR do?

When the user has entered text in the prompt input in web duo and web agentic duo, use localstorage to hold on to that prompt draft. This allows for preserving the draft when the user switches from agentic to non-agentic chat and if the user navigates from one page or another or refreshes the page.

This MR also adds failsafe wrappers for reading and writing localStorage, just in case it is not available.

Relates to Preserve User Input When Switching Between Duo ... (gitlab#579295)

Screenshots or screen recordings

Before	After
Screen Recording 2025-12-11 at 11.19.10.mov	Screen Recording 2025-12-11 at 11.14.51.mov

To test

1. Use this branch of duo-ui locally in the GDK
2. write a message to Duo and do not submit it
3. toggle between agentic and non-agentic chat
4. your draft will be preserved
5. If you use devtools to look at localstorage, you will see a gl-duo-prompt-draft key with a string that matches your draft input.
6. submit your message
7. the prompt input will be blank as expected after submitting
8. the localstorage key will be cleared out

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

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