Use hash-based routing to keep base URL visible throughout app

[Copilot]

Summary

Switches from BrowserRouter to HashRouter to keep base URL visible. Previously URLs showed http://localhost:9090/login and http://localhost:9090/app during navigation. Now routes appear after hash fragment: http://localhost:9090/#/login, http://localhost:9090/#/app.

Changes

• InvokeAIUI.tsx: Replace BrowserRouter with HashRouter
• LoginPage.tsx: Update redirect to window.location.href = '/#/app'
• AdministratorSetup.tsx: Update redirect to window.location.href = '/#/login'

Uses window.location.href instead of window.location.hash to maintain full page reloads for multiuser isolation.

Related Issues / Discussions

None

QA Instructions

1. Start app on feature/multiuser branch with multiuser enabled
2. Navigate to login - URL should show http://localhost:9090/#/login
3. Complete login - URL should show http://localhost:9090/#/app with page reload
4. Test admin setup flow - redirects to /#/login with page reload
5. Verify browser back/forward navigation works correctly
6. Verify direct URL access (e.g., http://localhost:9090/#/login) works

Merge Plan

None

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.