M1: implement operational core

Demo auth, seed import/reset, dashboard, vehicle/booking list+detail, audit trail. Backend: 19 tests passing, ruff clean. Frontend: React Router shell, typed API client, responsive pages. Verified end-to-end via curl and browser.

backend/app/core/errors.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+import uuid
+from typing import Any
+
+
+class AppError(Exception):
+    def __init__(
+        self,
+        code: str,
+        message: str,
+        status_code: int = 400,
+        details: dict[str, Any] | None = None,
+    ) -> None:
+        super().__init__(message)
+        self.code = code
+        self.message = message
+        self.status_code = status_code
+        self.details = details or {}
+        self.correlation_id = str(uuid.uuid4())
+
+
+def error_body(code: str, message: str, correlation_id: str, details: dict[str, Any]) -> dict:
+    return {
+        "error": {
+            "code": code,
+            "message": message,
+            "correlation_id": correlation_id,
+            "details": details,
+        }
+    }