Add user password change functionality

Introduced a backend API endpoint for changing user passwords with validation. Added a new "User Settings" page in the frontend to allow users to update their passwords, including a matching UI update for navigation and styles.

backend/app/schemas/user.py

@@ -1,5 +1,5 @@
 from datetime import datetime
-from pydantic import BaseModel, EmailStr
+from pydantic import BaseModel, EmailStr, field_validator
 
 
 class UserOut(BaseModel):
@@ -21,3 +21,15 @@ class UserUpdate(BaseModel):
     email: EmailStr | None = None
     password: str | None = None
     role: str | None = None
+
+
+class UserPasswordChange(BaseModel):
+    current_password: str
+    new_password: str
+
+    @field_validator("new_password")
+    @classmethod
+    def validate_new_password(cls, value: str) -> str:
+        if len(value) < 8:
+            raise ValueError("new_password must be at least 8 characters")
+        return value