MDL-78217 grade: Refine front-end weight handling and add validation

In this commit, the front-end experience for grade item weights in the
gradebook setup page is refined, building upon the improvements made in
the previous commit.

Firstly, overridden weights are excluded from automatic adjustments.
This enhancement guarantees that user-specified weights remain unchanged
by front-end calculations, thereby preserving the flexibility of custom
weight assignments.

Additionally, front-end form validation has been introduced to enhance
the user experience. Now, users will see validation errors if they
submit invalid values, giving them the opportunity to correct the values
themselves. This ensures that weights are not automatically normalised
by the back-end code, preserving the values as entered by the user.