IA-955 : feat: Improve MatiereDialog with error feedback, niveauId guard, and ApprentissageView code cleanup

Summary

Improves the MatiereDialog with proper error/warning feedback and niveauId validation, and cleans up handler functions in ApprentissageView.

Changes

MatiereDialog

• Added submitError state with inline <Alert severity="error"> displayed inside the dialog instead of silently failing
• Added niveauId validation guard: shows a <Alert severity="warning"> banner when no niveau is selected, and blocks submission with a user-facing error message
• Replaced onClose() calls with a handleClose() wrapper that resets submitError on dismiss
• Error message is now contextual: different wording for add vs edit mode

ApprentissageView

• Fixed navigateToMatieres to use niveau.uuid instead of niveau.levelUuid for setting currentNiveauId
• Cleaned up handleDeleteNiveauRows, handleDeleteMatiereRows, handleDeleteChapitreRows, handleDeleteExerciceRows: removed redundant comments, simplified to direct await + refetch pattern
• Cleaned up handleToggleMatiereActive, handleToggleChapitreActive, handleToggleActiveExercice: removed verbose comments and tightened handler bodies