Give theme cookie an expiration

[JamesNK]

Description

I noticed that Aspire dashboard sometimes lost the theme. The reason is the theme cookie had a session lifetime. Once all browsers are closed the cookie is cleared and the dashboard forgets the configured theme.

This PR:

• Gives the cookie a 1 year expiration
• If the theme cookie is present when the dashboard loads, it is set again to create a sliding expiration (i.e. cookie expires 1 year after the dashboard was last loaded)
• Specifies a path when working with the cookie to be explicit about where it is set
• Deletes the cookie if "System" is selected in the UI. No point having a cookie set and updating it if it is the default value.

Fixes #5430

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire-docs issue:
  • No

Microsoft Reviewers: Open in CodeFlow