IAV-1209: integration config UX overhaul, step skeletons & unsaved changes warning

• Integration config account management rewrite — Unified the account management section into a consistent 4-scenario layout:
  • No configured account + no existing accounts → info message + "Créer un nouveau compte"
  • No configured account + existing accounts → Select → "Tester la connexion" (appears on selection) → "Créer un nouveau compte"
  • Configured account → [Select if ≥2 accounts] → [Tester la connexion if different selection] → RecapCard → Divider → "Créer un nouveau compte"
  • After successful test → RecapCard only (test button and success message hidden)
• confirmedAccounts state — Separates "currently selected in dropdown" from "confirmed/saved account" to prevent premature case switching and control when the test button appears
• Step skeletons (wizard-step-skeleton.tsx) — New file with per-step skeleton loaders (welcome, topics, integrations, config, resume) matching the aqal-wizard pattern, including a submitting dots overlay with step-specific labels
• Unsaved changes warning — Warning banner "Sans cliquer sur Suivant, vos configurations ne seront pas sauvegardées." appears on steps 2–4 when local state differs from the last saved snapshot; resets after successful save
• Summary step navigation — Replaced the centered "Créer l'agent" button with the shared NavigationButtons component, adding a "Précédent" button on the left