[Tests] Add missing test coverage for critical paths

[ibourgeois]

Missing Coverage

The following scenarios have no test coverage:

1. Impersonation real session flow: existing test re-auths before DELETE, masking the escape-hatch bug. Need a test that mirrors what a browser session does.
2. Double propose: calling POST /ideas/{idea}/propose twice should result in exactly one project, not two.
3. Propose with null fields: an idea with null summary/details should not crash on propose.
4. Unauthenticated redirects: no test verifies that /ideas, /notifications, and admin routes redirect unauthenticated users to /login.
5. Idea user isolation: no test verifies a user cannot access another user's idea (edit, update, delete).
6. Non-admin blocked from admin routes: GET /admin/users, GET /admin/users/{user}/edit, PUT /admin/users/{user}.
7. Registration edge cases: duplicate email, missing required fields, password confirmation mismatch.

Files

• tests/Feature/Admin/ImpersonationTest.php
• tests/Feature/IdeaProposalTest.php (new)
• tests/Feature/AuthRedirectTest.php (new)
• tests/Feature/IdeaIsolationTest.php (new)
• tests/Feature/Admin/UserControllerTest.php (new)
• tests/Feature/Auth/RegistrationTest.php