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/app/src/error/user.rs

@@ -1,12 +1,22 @@
 #[derive(Debug)]
 pub enum UserError {
     NotFound,
+    NotFollowing,
+    Forbidden,
+    UsernameTaken,
+    AlreadyFollowing,
+    Internal(String),
 }
 
 impl std::fmt::Display for UserError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             UserError::NotFound => write!(f, "User not found"),
+            UserError::NotFollowing => write!(f, "You are not following this user"),
+            UserError::Forbidden => write!(f, "You do not have permission to perform this action"),
+            UserError::UsernameTaken => write!(f, "Username is already taken"),
+            UserError::AlreadyFollowing => write!(f, "You are already following this user"),
+            UserError::Internal(msg) => write!(f, "Internal server error: {}", msg),
         }
     }
 }