feat: Hybride Passkey-Freigabe für Skipper und Crew

Skipper (nur Owner) und Crew (WRITE-Collaborators) können Logbuchseiten
optional per WebAuthn freigeben; klassische Unterschrift bleibt als Fallback.
Signatur ist an den Eintrags-Hash gebunden, Export in CSV/PDF angepasst.

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

client/src/utils/signatures.ts

@@ -1,12 +1,50 @@
+import type { PasskeySignature, SignatureValue } from '../types/signatures.js'
+
 export function isSignatureImage(value: string | undefined | null): boolean {
   return typeof value === 'string' && value.startsWith('data:image/')
 }
 
-export function formatSignatureForExport(
-  value: string | undefined | null,
+export function isPasskeySignature(value: unknown): value is PasskeySignature {
+  return (
+    typeof value === 'object' &&
+    value !== null &&
+    (value as PasskeySignature).kind === 'passkey' &&
+    (value as PasskeySignature).version === 1
+  )
+}
+
+export function normalizeSignature(value: unknown): SignatureValue | undefined {
+  if (value === null || value === undefined || value === '') return undefined
+  if (isPasskeySignature(value)) return value
+  if (typeof value === 'string') return value
+  return undefined
+}
+
+export function isSignatureValidForEntry(sig: PasskeySignature, entryHash: string): boolean {
+  return sig.entryHash === entryHash
+}
+
+export interface SignatureExportLabels {
   imagePlaceholder: string
+  passkeyLabel: (username: string, signedAt: string) => string
+}
+
+export function formatSignatureForExport(
+  value: SignatureValue | undefined | null,
+  labels: SignatureExportLabels
 ): string {
   if (!value) return ''
-  if (isSignatureImage(value)) return imagePlaceholder
+  if (isPasskeySignature(value)) {
+    return labels.passkeyLabel(value.username, value.signedAt)
+  }
+  if (isSignatureImage(value)) return labels.imagePlaceholder
   return value
 }
+
+export function serializeSignature(value: SignatureValue | '' | undefined): SignatureValue | undefined {
+  if (!value) return undefined
+  if (isPasskeySignature(value)) return value
+  if (isSignatureImage(value)) return value
+  const trimmed = value.trim()
+  return trimmed || undefined
+}