feat(accounting): frontend rework - separate Incoming invoices vs Expenses (stage 2)

Matches the backend split. Incoming invoices and Expenses are now distinct
surfaces with no shared rows.

Incoming invoices (AccountingInboxPage): triage sets the disposition + booking
(event or company) ON the document; "Mark paid" / "Paid" toggle records
supplier payment HERE with the outstanding total shown; re-bill via the
customer picker + markup. PDF preview still rasterised (last page = QR-bill).

Expenses (ExpensesLedgerPage): internal own-costs only. Add form has a Type
dropdown (amount / mileage(km) / per-diem); km/per-diem switch the input to a
quantity + rate (default from accounting settings, per-entry override) with a
live computed amount; optional proof upload (required when the setting says so);
localized category; booked to an event or the company. Proof viewable per row.

Service: reworked to the new endpoints/shapes; categoryLabel() localizes seed
categories (custom stay free-text). i18n: accounting.booking / incoming /
expense / expenseKind / category (EN + DE, DE native).

Verified: tsc --noEmit clean (0 errors); en/de JSON valid; npm run build green.

frontend/src/i18n/locales/de.json

@@ -3475,10 +3475,57 @@
       "card": "Karte",
       "other": "Sonstiges"
     },
+    "booking": {
+      "label": "Buchen auf",
+      "company": "Firma",
+      "event": "Event",
+      "eventId": "Event-ID"
+    },
+    "incoming": {
+      "triageTitle": "Eingangsrechnung kategorisieren",
+      "payTitle": "Lieferant als bezahlt markieren",
+      "outstanding": "Offen",
+      "markPaid": "Als bezahlt markieren",
+      "confirmPaid": "Als bezahlt markieren",
+      "paid": "Bezahlt",
+      "paidToast": "Als bezahlt markiert.",
+      "categorizedToast": "Kategorisiert."
+    },
+    "expense": {
+      "kind": "Art",
+      "km": "Kilometer",
+      "days": "Tage",
+      "rate": "Satz",
+      "rateDefault": "Standard {{rate}} — leer lassen, um ihn zu verwenden",
+      "computed": "Betrag",
+      "who": "Bezahlt von / Lieferant",
+      "whoHint": "z. B. Mitarbeitername oder Geschäft",
+      "proof": "Beleg",
+      "proofRequired": "Ein Beleg ist erforderlich.",
+      "viewProof": "Beleg"
+    },
+    "expenseKind": {
+      "amount": "Betrag",
+      "mileage": "Kilometer (km)",
+      "per_diem": "Spesenpauschale"
+    },
+    "category": {
+      "infrastructure": "Infrastruktur & Miete",
+      "equipment": "Equipment & Hardware",
+      "software": "Software & Lizenzen",
+      "material": "Material & Verbrauch",
+      "travel": "Reise & Spesen",
+      "marketing": "Werbung & Marketing",
+      "services": "Dienstleistungen/Fremdleistungen",
+      "insurance": "Versicherungen & Gebühren",
+      "training": "Weiterbildung",
+      "other": "Sonstiges"
+    },
     "ledger": {
       "allStatuses": "Alle Status",
       "allDispositions": "Alle Zuordnungen",
-      "empty": "Noch keine Aufwände — Dokumente im Eingang kategorisieren.",
+      "allKinds": "Alle Arten",
+      "empty": "Noch keine Aufwände — oben einen hinzufügen.",
       "untitled": "Aufwand",
       "invoiceLink": "Rechnung",
       "paid": "Bezahlt",

frontend/src/i18n/locales/en.json

@@ -3475,10 +3475,57 @@
       "card": "Card",
       "other": "Other"
     },
+    "booking": {
+      "label": "Book to",
+      "company": "Company",
+      "event": "Event",
+      "eventId": "Event ID"
+    },
+    "incoming": {
+      "triageTitle": "Categorize incoming invoice",
+      "payTitle": "Mark supplier paid",
+      "outstanding": "Outstanding",
+      "markPaid": "Mark paid",
+      "confirmPaid": "Mark paid",
+      "paid": "Paid",
+      "paidToast": "Marked as paid.",
+      "categorizedToast": "Categorized."
+    },
+    "expense": {
+      "kind": "Type",
+      "km": "Kilometres",
+      "days": "Days",
+      "rate": "Rate",
+      "rateDefault": "Default {{rate}} — leave blank to use it",
+      "computed": "Amount",
+      "who": "Paid by / vendor",
+      "whoHint": "e.g. coworker name or shop",
+      "proof": "Proof",
+      "proofRequired": "A proof file is required.",
+      "viewProof": "Proof"
+    },
+    "expenseKind": {
+      "amount": "Amount",
+      "mileage": "Mileage (km)",
+      "per_diem": "Per-diem"
+    },
+    "category": {
+      "infrastructure": "Infrastructure & rent",
+      "equipment": "Equipment & hardware",
+      "software": "Software & licenses",
+      "material": "Materials & supplies",
+      "travel": "Travel & expenses",
+      "marketing": "Advertising & marketing",
+      "services": "Services / subcontracting",
+      "insurance": "Insurance & fees",
+      "training": "Training",
+      "other": "Other"
+    },
     "ledger": {
       "allStatuses": "All statuses",
       "allDispositions": "All dispositions",
-      "empty": "No expenses yet — categorize documents in the inbox.",
+      "allKinds": "All types",
+      "empty": "No expenses yet — add one above.",
       "untitled": "Expense",
       "invoiceLink": "Invoice",
       "paid": "Paid",