feat: initialize thoughts-frontend with Next.js, TypeScript, and ESLint

- Add ESLint configuration for Next.js and TypeScript support.
- Create Next.js configuration file with standalone output option.
- Initialize package.json with scripts for development, build, and linting.
- Set up PostCSS configuration for Tailwind CSS.
- Add SVG assets for UI components.
- Create TypeScript configuration for strict type checking and module resolution.

thoughts-backend/api/src/validation/mod.rs

@@ -0,0 +1,3 @@
+mod rejection;
+
+pub use rejection::ValidRejection;

thoughts-backend/api/src/validation/rejection.rs

@@ -0,0 +1,58 @@
+use axum::http::StatusCode;
+use axum::response::{IntoResponse, Response};
+use validator::ValidationErrors;
+
+use crate::models::ValidationErrorResponse;
+
+#[derive(Debug)]
+pub enum ValidationRejection<V, E> {
+    Validator(V), // Validation errors
+    Extractor(E), // Extraction errors, e.g. axum's JsonRejection
+}
+
+impl<V: std::fmt::Display, E: std::fmt::Display> std::fmt::Display for ValidationRejection<V, E> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ValidationRejection::Validator(v) => write!(f, "{v}"),
+            ValidationRejection::Extractor(e) => write!(f, "{e}"),
+        }
+    }
+}
+
+impl<V: std::error::Error + 'static, E: std::error::Error + 'static> std::error::Error
+    for ValidationRejection<V, E>
+{
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            ValidationRejection::Validator(v) => Some(v),
+            ValidationRejection::Extractor(e) => Some(e),
+        }
+    }
+}
+
+impl<V: serde::Serialize + std::error::Error, E: IntoResponse> IntoResponse
+    for ValidationRejection<V, E>
+{
+    fn into_response(self) -> Response {
+        match self {
+            ValidationRejection::Validator(v) => {
+                tracing::error!("Validation error: {v}");
+                (
+                    StatusCode::UNPROCESSABLE_ENTITY,
+                    axum::Json(ValidationErrorResponse::from(v)),
+                )
+                    .into_response()
+            }
+            // logged by ApiError
+            ValidationRejection::Extractor(e) => e.into_response(),
+        }
+    }
+}
+
+pub type ValidRejection<E> = ValidationRejection<ValidationErrors, E>;
+
+impl<E> From<ValidationErrors> for ValidRejection<E> {
+    fn from(v: ValidationErrors) -> Self {
+        Self::Validator(v)
+    }
+}