feat(accounting): incoming-invoices inbox with camera capture + triage/re-bill

Adds the Accounting → Incoming invoices frontend on top of the existing
/api/admin/expenses backend:

- accounting.service.ts: typed client (inbound upload/list/get/update/
  categorize, expense list, re-bill, supplier-payment, categories).
- AccountingInboxPage: capture a supplier invoice via the device CAMERA
  (<input accept="image/*" capture="environment">) or a PDF/image upload;
  inbox list with status badges + parsed summary; a triage modal to confirm
  fields and pick a disposition (re-bill / pass-through / company expense /
  duplicate / declined). Re-bill uses the customer picker and mints an
  editable scheduled invoice (chains categorize -> rebill).
- AccountingLayout: "Incoming invoices" sub-nav item + AccountingIndex that
  redirects /admin/accounting to the first enabled sub-feature.
- App.tsx: /admin/accounting/inbox route (gated by incomingInvoices).
- i18n: accounting.inbox/disposition/markup + subnav.incomingInvoices +
  common.saving (EN + DE, DE authored natively).

Camera capture needs no native app — the mobile web input drives the device
camera straight into the upload endpoint. OCR/QR auto-extraction is still a
backend follow-up (extractionService is a no-op), so fields are confirmed
manually in the triage modal for now.

Verified: npm run build green; en/de JSON valid.

frontend/src/i18n/locales/de.json

@@ -98,6 +98,7 @@
     "error": "Fehler",
     "configureInSettings": "Standards in den Einstellungen anpassen ↗",
     "save": "Speichern",
+    "saving": "Speichern…",
     "cancel": "Abbrechen",
     "delete": "Löschen",
     "edit": "Bearbeiten",
@@ -3398,7 +3399,54 @@
       "body": "Aktiviere die Steuerliste (oder eine andere Buchhaltungs-Unterfunktion) unter Einstellungen → Funktionen, um loszulegen."
     },
     "subnav": {
+      "incomingInvoices": "Eingangsrechnungen",
       "taxReport": "Steuer"
+    },
+    "disposition": {
+      "rebill": "An Kunde weiterverrechnen",
+      "durchlaufend": "Durchlaufender Posten",
+      "eigener_aufwand": "Eigener Aufwand",
+      "duplikat": "Duplikat",
+      "abgelehnt": "Abgelehnt"
+    },
+    "markup": {
+      "none": "Keiner / aus Vertrag",
+      "percent": "Prozent",
+      "flat": "Pauschal"
+    },
+    "inbox": {
+      "captureTitle": "Lieferantenrechnung erfassen",
+      "captureBody": "Fotografiere eine Papierrechnung mit der Gerätekamera oder lade ein PDF / Bild hoch.",
+      "scanCamera": "Mit Kamera scannen",
+      "uploadFile": "Datei hochladen",
+      "capturedToast": "Dokument erfasst.",
+      "categorizedToast": "Dokument kategorisiert.",
+      "triageTitle": "Dokument kategorisieren",
+      "saveCategorize": "Speichern",
+      "categorize": "Kategorisieren",
+      "empty": "Noch keine Dokumente — oben eines erfassen.",
+      "untitled": "Unbenanntes Dokument",
+      "noAmount": "Betrag nicht erfasst",
+      "rebillHint": "Erstellt eine bearbeitbare geplante Rechnung beim Kunden. MwSt-/Steuerbehandlung ist v1 — mit Treuhänder prüfen.",
+      "status": {
+        "unsorted": "Neu",
+        "categorized": "Kategorisiert",
+        "declined": "Abgelehnt",
+        "duplicate": "Duplikat"
+      },
+      "field": {
+        "supplier": "Lieferant",
+        "total": "Total",
+        "currency": "Währung",
+        "invoiceDate": "Rechnungsdatum",
+        "disposition": "Zuordnung",
+        "category": "Kategorie",
+        "categoryNone": "— keine —",
+        "declineReason": "Grund",
+        "customer": "Kunde",
+        "eventId": "Event-ID (optional)",
+        "markup": "Zuschlag"
+      }
     }
   },
   "calendar": {

frontend/src/i18n/locales/en.json

@@ -98,6 +98,7 @@
     "error": "Error",
     "configureInSettings": "Configure defaults in Settings ↗",
     "save": "Save",
+    "saving": "Saving…",
     "cancel": "Cancel",
     "delete": "Delete",
     "edit": "Edit",
@@ -3398,7 +3399,54 @@
       "body": "Enable the Tax report (or another accounting sub-feature) under Settings → Features to get started."
     },
     "subnav": {
+      "incomingInvoices": "Incoming invoices",
       "taxReport": "Tax"
+    },
+    "disposition": {
+      "rebill": "Re-bill to client",
+      "durchlaufend": "Pass-through",
+      "eigener_aufwand": "Company expense",
+      "duplikat": "Duplicate",
+      "abgelehnt": "Declined"
+    },
+    "markup": {
+      "none": "None / from contract",
+      "percent": "Percent",
+      "flat": "Flat"
+    },
+    "inbox": {
+      "captureTitle": "Capture a supplier invoice",
+      "captureBody": "Photograph a paper invoice with your device camera, or upload a PDF / image.",
+      "scanCamera": "Scan with camera",
+      "uploadFile": "Upload file",
+      "capturedToast": "Document captured.",
+      "categorizedToast": "Document categorized.",
+      "triageTitle": "Categorize document",
+      "saveCategorize": "Save",
+      "categorize": "Categorize",
+      "empty": "No documents yet — capture one above.",
+      "untitled": "Untitled document",
+      "noAmount": "amount not entered",
+      "rebillHint": "Creates an editable scheduled invoice on the client. VAT/tax handling is v1 — verify with your Treuhänder.",
+      "status": {
+        "unsorted": "New",
+        "categorized": "Categorized",
+        "declined": "Declined",
+        "duplicate": "Duplicate"
+      },
+      "field": {
+        "supplier": "Supplier",
+        "total": "Total",
+        "currency": "Currency",
+        "invoiceDate": "Invoice date",
+        "disposition": "Disposition",
+        "category": "Category",
+        "categoryNone": "— none —",
+        "declineReason": "Reason",
+        "customer": "Client",
+        "eventId": "Event ID (optional)",
+        "markup": "Markup"
+      }
     }
   },
   "calendar": {