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/models.py

@@ -40,6 +40,9 @@ class Permission(str, PyEnum):
     # Audit permissions
     VIEW_AUDIT = "view_audit"
 
+    # Profile permissions
+    MANAGE_OWN_PROFILE = "manage_own_profile"
+
     # Invite permissions
     MANAGE_INVITES = "manage_invites"
     VIEW_OWN_INVITES = "view_own_invites"
@@ -93,6 +96,7 @@ ROLE_DEFINITIONS: dict[str, RoleConfig] = {
             Permission.VIEW_COUNTER,
             Permission.INCREMENT_COUNTER,
             Permission.USE_SUM,
+            Permission.MANAGE_OWN_PROFILE,
             Permission.VIEW_OWN_INVITES,
             Permission.BOOK_APPOINTMENT,
             Permission.VIEW_OWN_APPOINTMENTS,