Persist selected users to localStorage

[MathijsR94]

Summary

• Persist user settings (users, org, date range, visible stats) to localStorage so they are restored when revisiting the app without a URL state parameter
• URL state still takes priority — localStorage is only used as a fallback
• Settings are cleared from storage when all values are empty

QA instructions

• Load the app, add some users, then close and reopen the tab (without a ?state= param) — settings should be restored
• Verify that a ?state= URL param still takes priority over stored settings
• Clear all users/settings and confirm localStorage entry is removed