feat: Rechnungen, Dokumente und Preview im Ticket buendeln

- InvoiceNinja-Rechnungen pro Kunde/Ticket anzeigen, verknuepfen und aus
  Ticket heraus erstellen (integrationsApi, useInvoices, InvoicePanel)
- Paperless-ngx: Ticket-Uploads automatisch synchronisieren und Kunden-/
  Ticket-Dokumente durchsuchen/anzeigen (DocumentsPanel, FileUploadModal)
- Projekt-/Preview-Karte mit Status, Provisioning und letztem Commit
- Einheitliches Ticket-Detail mit Tabs (Details/Arbeitsblaetter/Rechnungen/
  Dokumente/Projekt) + zugehoerige Styles

Aufruf des Integrations-Backends same-origin per Appwrite-JWT.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/hooks/useInvoices.js

@@ -0,0 +1,49 @@
+import { useState, useEffect, useCallback } from 'react'
+import { integrationsApi } from '../lib/integrationsApi'
+
+/**
+ * Laedt die InvoiceNinja-Rechnungen eines verknuepften Clients.
+ * clientId = customers.invoiceNinjaClientId (oder null -> nichts laden).
+ */
+export function useInvoices(clientId) {
+  const [invoices, setInvoices] = useState([])
+  const [loading, setLoading] = useState(false)
+  const [error, setError] = useState(null)
+
+  const fetchInvoices = useCallback(async () => {
+    if (!clientId) {
+      setInvoices([])
+      return
+    }
+    setLoading(true)
+    setError(null)
+    try {
+      const data = await integrationsApi.listInvoices(clientId)
+      setInvoices(data.invoices || [])
+    } catch (err) {
+      setError(err.message)
+      setInvoices([])
+    } finally {
+      setLoading(false)
+    }
+  }, [clientId])
+
+  useEffect(() => {
+    fetchInvoices()
+  }, [fetchInvoices])
+
+  const createInvoice = useCallback(
+    async (payload) => {
+      try {
+        const data = await integrationsApi.createInvoice(payload)
+        await fetchInvoices()
+        return { success: true, invoice: data.invoice }
+      } catch (err) {
+        return { success: false, error: err.message }
+      }
+    },
+    [fetchInvoices]
+  )
+
+  return { invoices, loading, error, refresh: fetchInvoices, createInvoice }
+}