refactor(auth): unify authorization patterns with MANAGE_OWN_PROFILE permission

Issue #2: The profile route used a custom role-based check instead
of the permission-based pattern used everywhere else.

Changes:
- Add MANAGE_OWN_PROFILE permission to backend Permission enum
- Add permission to ROLE_REGULAR role definition
- Update profile routes to use require_permission(MANAGE_OWN_PROFILE)
- Remove custom require_regular_user dependency
- Update frontend Permission constant and profile page
- Update invites page to use permission instead of role check
- Update profile tests with proper permission mocking

This ensures consistent authorization patterns across all routes.

backend/tests/test_profile.py

@@ -166,12 +166,12 @@ class TestGetProfileEndpoint:
         assert data["nostr_npub"] is None
 
     async def test_admin_user_cannot_access_profile(self, client_factory, admin_user):
-        """Admin user gets 403 when trying to access profile."""
+        """Admin user gets 403 when trying to access profile (lacks MANAGE_OWN_PROFILE)."""
         async with client_factory.create(cookies=admin_user["cookies"]) as client:
             response = await client.get("/api/profile")
 
         assert response.status_code == 403
-        assert "regular users" in response.json()["detail"].lower()
+        assert "manage_own_profile" in response.json()["detail"].lower()
 
     async def test_unauthenticated_user_gets_401(self, client_factory):
         """Unauthenticated user gets 401."""