fix: narrow type for PasswordChange to satisfy type checker

Use isinstance check instead of bool flag to help ty resolve
the current_password attribute on the validated model.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/porchlight/manage/routes.py

@@ -82,7 +82,7 @@ async def set_password(
         return HTMLResponse(format_validation_errors(exc))
 
     # Verify current password if changing
-    if has_password:
+    if has_password and isinstance(validated, PasswordChange):
         if not password_service.verify(existing.password_hash, validated.current_password):
             return HTMLResponse('<div role="alert">Current password is incorrect</div>')