refactor(client): unify wizard form plumbing

Extract getSubmitErrors and cleanServerSection into lib so the three
section wizards stop duplicating submit-error parsing and server-section
cleaning. Mount UnsavedChangesDialog in the questionnaire and CV wizards
and delete their hand-rolled beforeunload listeners, so every multi-
section surface guards in-app and hard navigation through one component
fed by the canonical value-based isDirty. Simplify the employee profile
tab-change save now that persistSection self-guards.