Persist docs preference cookies across browser sessions

[SeppahBaws]

Previously the preferences such as language and theme were stored as session cookies, which means they got cleared whenever the browser shut down. Adding the max-age makes sure that they stay if you close the browser.

To help everyone out, please make sure your PR does the following:

• [ ] Update the first line to point to the ticket that this PR fixes
• [ ] Add a message that clearly describes the fix
• [ ] If applicable, add a test that would fail without this fix
• [ ] Make sure the unit and integration tests pass locally with pnpm run tests and cd integration && pnpm run tests
• [ ] Includes a changeset if your fix affects the user with pnpm changeset