Replace logbook backup v1 JSON with v2 ZIP archives.

ZIP .daagbok files use a compact manifest and binary KDAB blobs so large
photo, voice, and GPS payloads no longer inflate in a single JSON file.

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

client/src/services/logbookBackup/encBlob.ts

@@ -0,0 +1,45 @@
+import { base64ToBuffer, bufferToBase64 } from '../crypto.js'
+
+export const ENC_MAGIC = new Uint8Array([0x4b, 0x44, 0x41, 0x42]) // KDAB
+export const ENC_FORMAT_VERSION = 1
+export const ENC_HEADER_SIZE = 33 // 4 + 1 + 12 + 16
+
+export interface DexieEncFields {
+  encryptedData: string
+  iv: string
+  tag: string
+}
+
+export function encBytesFromDexieFields(fields: DexieEncFields): Uint8Array {
+  const iv = new Uint8Array(base64ToBuffer(fields.iv))
+  const tag = new Uint8Array(base64ToBuffer(fields.tag))
+  const ciphertext = new Uint8Array(base64ToBuffer(fields.encryptedData))
+  if (iv.length !== 12) throw new Error('BACKUP_INVALID_ENC')
+  if (tag.length !== 16) throw new Error('BACKUP_INVALID_ENC')
+
+  const out = new Uint8Array(ENC_HEADER_SIZE + ciphertext.length)
+  out.set(ENC_MAGIC, 0)
+  out[4] = ENC_FORMAT_VERSION
+  out.set(iv, 5)
+  out.set(tag, 17)
+  out.set(ciphertext, 33)
+  return out
+}
+
+export function dexieFieldsFromEncBytes(bytes: Uint8Array): DexieEncFields {
+  if (bytes.length < ENC_HEADER_SIZE) throw new Error('BACKUP_INVALID_ENC')
+  for (let i = 0; i < 4; i++) {
+    if (bytes[i] !== ENC_MAGIC[i]) throw new Error('BACKUP_INVALID_ENC')
+  }
+  if (bytes[4] !== ENC_FORMAT_VERSION) throw new Error('BACKUP_INVALID_ENC')
+
+  const iv = bufferToBase64(bytes.slice(5, 17).buffer)
+  const tag = bufferToBase64(bytes.slice(17, 33).buffer)
+  const ciphertext = bufferToBase64(bytes.slice(33).buffer)
+  return { encryptedData: ciphertext, iv, tag }
+}
+
+export function encByteLength(fields: DexieEncFields): number {
+  const ct = base64ToBuffer(fields.encryptedData).byteLength
+  return ENC_HEADER_SIZE + ct
+}