Fix annotation guide unsaved changes & back-navigation

[prathamesh-patil-5090]

PR: Fix annotation guide unsaved changes & back-navigation

Summary

This PR addresses two related user experience issues in the annotation guide editor:

1. Unsaved changes vulnerability

   • The editor now auto-saves text to browser localStorage as a draft.
   • Draft is restored on revisit and a notification offers to discard it.
   • In-app navigation is blocked via <Prompt> when unsaved changes exist.
   • A beforeunload handler warns on browser refresh or tab close.
   • Draft is cleared after a successful submit.

2. Incorrect back-button target

   • The "Back" button on the guide page previously used prevLocation from Redux,
     which defaulted to /tasks and /projects regardless of the current item.
   • Now it explicitly navigates to /tasks/<id> or /projects/<id> based on the
     current URL. The unsaved-changes prompt still intercepts this navigation.

Testing steps

1. Open a task or project and navigate to its annotation guide.
2. Type text but don’t submit. Try navigating away; confirm the prompt appears.
3. Confirm draft restoration after refresh, and discard functionality works.
4. Click the back button and verify it sends you to /tasks/<id> or /projects/<id>.
5. Repeat with unsaved changes to ensure the confirmation dialog appears.

Related issue

Closes #10303 (extracted from #6536).

Additional notes

• The annotation-guide-page component was updated extensively; please review carefully.
• A new changelog fragment has been added at changelog.d/20260308_annotation_guide_draft.md.

Test

Screencast.from.2026-03-09.00-15-09.webm