feat(setup): add "How will you use PicPeak?" feature-selection step

After the admin account is created (and we're logged in), the wizard now
shows an opt-in feature step instead of jumping straight to the dashboard.
Grouped ticks (Client management / Accounting / Automation) map to the
existing feature flags; galleries/analytics/userManagement stay always-on
and are noted, not listed.

- Selection is saved via the existing authenticated PUT /admin/feature-flags,
  whose server-side applyDependencyRules resolves dependencies (e.g. Invoices
  pulls in Accounting) — the wizard only sends raw ticks.
- Labels/descriptions reuse settings.features.<key>.title/description so
  translations stay in sync (en + de verified for all 14 features).
- Saving is best-effort: on failure the admin still enters the app and can
  set features later in Settings.
- New en/de strings for the usage step.

Option A (lean wizard): this is the feature-selection foundation; per-feature
hard-required config steps + the restore-from-backup branch come next.

frontend/src/i18n/locales/de.json

@@ -3467,6 +3467,15 @@
     "subtitle": "Erstellen Sie Ihr Administrator-Konto, um loszulegen",
     "tokenStepSubtitle": "Geben Sie zunächst Ihren einmaligen Setup-Token ein",
     "accountStepSubtitle": "Erstellen Sie nun Ihr Administrator-Konto",
+    "usageSubtitle": "Wie möchten Sie PicPeak nutzen?",
+    "usageAlwaysOn": "Galerien, Analysen und Benutzerverwaltung sind immer enthalten. Wählen Sie unten optionale Funktionen — Sie können dies jederzeit in den Einstellungen ändern.",
+    "usageGroupCrm": "Kundenverwaltung",
+    "usageGroupAccounting": "Buchhaltung",
+    "usageGroupAutomation": "Automatisierung & Versand",
+    "usageDepsNote": "Rechnungen aktivieren automatisch den Buchhaltungsbereich.",
+    "usageSkip": "Nur mit Galerien fortfahren",
+    "finish": "Einrichtung abschließen",
+    "featuresSaveFailed": "Ihre Funktionsauswahl konnte nicht gespeichert werden — Sie können sie später unter Einstellungen → Funktionen festlegen.",
     "stepOf": "Schritt {{current}} von {{total}}",
     "continue": "Weiter",
     "back": "Zurück",

frontend/src/i18n/locales/en.json

@@ -3363,6 +3363,15 @@
     "subtitle": "Create your administrator account to get started",
     "tokenStepSubtitle": "First, enter your one-time setup token",
     "accountStepSubtitle": "Now create your administrator account",
+    "usageSubtitle": "How will you use PicPeak?",
+    "usageAlwaysOn": "Galleries, analytics and user management are always included. Pick any extras below — you can change these anytime in Settings.",
+    "usageGroupCrm": "Client management",
+    "usageGroupAccounting": "Accounting",
+    "usageGroupAutomation": "Automation & delivery",
+    "usageDepsNote": "Invoices automatically enable the Accounting area.",
+    "usageSkip": "Continue with galleries only",
+    "finish": "Finish setup",
+    "featuresSaveFailed": "Could not save your feature choices — you can set them later in Settings → Features.",
     "stepOf": "Step {{current}} of {{total}}",
     "continue": "Continue",
     "back": "Back",