Resolve "Provide optional title and description before submitting edits made with the Static Site Editor"

What does this MR do?

It:

1. Preserves the MR title and description in localStorage in the static site editor if the modal is accidentally or intentionally closed
2. Initializes w/the localStorage cache upon full page refresh or modal reopen during same session
3. Preselects the title input for one-click UX savings
4. Adds a "Keep editing" cancel button as per the latest design
5. Removes the corresponding localStorage key upon Submit changes

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #216861 (closed)