feat: implement user follow/unfollow functionality and thought retrieval by user

- Added follow and unfollow endpoints for users.
- Implemented logic to retrieve thoughts by a specific user.
- Updated user error handling to include cases for already following and not following.
- Created persistence layer for follow relationships.
- Enhanced user and thought schemas to support new features.
- Added tests for follow/unfollow endpoints and thought retrieval.
- Updated frontend to display thoughts and allow posting new thoughts.

thoughts-backend/api/src/error/adapter.rs

@@ -27,6 +27,11 @@ impl HTTPError for UserError {
     fn to_status_code(&self) -> StatusCode {
         match self {
             UserError::NotFound => StatusCode::NOT_FOUND,
+            UserError::NotFollowing => StatusCode::BAD_REQUEST,
+            UserError::Forbidden => StatusCode::FORBIDDEN,
+            UserError::UsernameTaken => StatusCode::BAD_REQUEST,
+            UserError::AlreadyFollowing => StatusCode::BAD_REQUEST,
+            UserError::Internal(_) => StatusCode::INTERNAL_SERVER_ERROR,
         }
     }
 }