Descarga de mangas completos a /zdata/media/mangas en CBZ

- Cola de descargas en segundo plano (se reanuda tras reinicios; repetirla
  solo baja capítulos nuevos; se puede cancelar).
- <Título>/Capitulo NNNN.cbz con ComicInfo.xml + cover.jpg (Kavita/Komga/Mihon).
- Los capítulos descargados se leen desde disco; sin conexión la ficha usa
  los datos guardados.
- Página "Descargas" y marcas ⬇ en la lista de capítulos.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/cbz.js

@@ -0,0 +1,115 @@
+// Lectura/escritura mínima de CBZ (ZIP) sin dependencias.
+// Escribe sin compresión (las imágenes ya vienen comprimidas); lee entradas "stored" y "deflate".
+import fs from 'node:fs'
+import zlib from 'node:zlib'
+
+const IMAGE_EXT = /\.(jpe?g|png|webp|gif|avif)$/i
+
+// entries: [{ name, data: Buffer }]. Escribe a un temporal y renombra para no dejar CBZ a medias.
+export function writeCbz(file, entries) {
+  const parts = []
+  const central = []
+  let offset = 0
+  for (const { name, data } of entries) {
+    const nameBuf = Buffer.from(name, 'utf8')
+    const crc = zlib.crc32(data)
+    const local = Buffer.alloc(30)
+    local.writeUInt32LE(0x04034b50, 0)
+    local.writeUInt16LE(20, 4) // versión necesaria
+    local.writeUInt16LE(0x0800, 6) // nombres en UTF-8
+    local.writeUInt16LE(0, 8) // sin compresión
+    local.writeUInt16LE(0, 10) // hora DOS
+    local.writeUInt16LE(0x21, 12) // fecha DOS (1980-01-01)
+    local.writeUInt32LE(crc, 14)
+    local.writeUInt32LE(data.length, 18)
+    local.writeUInt32LE(data.length, 22)
+    local.writeUInt16LE(nameBuf.length, 26)
+    local.writeUInt16LE(0, 28)
+    parts.push(local, nameBuf, data)
+
+    const cd = Buffer.alloc(46)
+    cd.writeUInt32LE(0x02014b50, 0)
+    cd.writeUInt16LE(20, 4)
+    cd.writeUInt16LE(20, 6)
+    cd.writeUInt16LE(0x0800, 8)
+    cd.writeUInt16LE(0, 10)
+    cd.writeUInt16LE(0, 12)
+    cd.writeUInt16LE(0x21, 14)
+    cd.writeUInt32LE(crc, 16)
+    cd.writeUInt32LE(data.length, 20)
+    cd.writeUInt32LE(data.length, 24)
+    cd.writeUInt16LE(nameBuf.length, 28)
+    cd.writeUInt32LE(offset, 42)
+    central.push(cd, nameBuf)
+    offset += 30 + nameBuf.length + data.length
+  }
+  const cdSize = central.reduce((n, b) => n + b.length, 0)
+  const end = Buffer.alloc(22)
+  end.writeUInt32LE(0x06054b50, 0)
+  end.writeUInt16LE(entries.length, 8)
+  end.writeUInt16LE(entries.length, 10)
+  end.writeUInt32LE(cdSize, 12)
+  end.writeUInt32LE(offset, 16)
+
+  const tmp = `${file}.part`
+  fs.writeFileSync(tmp, Buffer.concat([...parts, ...central, end]))
+  fs.renameSync(tmp, file)
+}
+
+// Devuelve las imágenes del CBZ ordenadas por nombre: [{ name, method, start, size }]
+export function readCbzIndex(file) {
+  const fd = fs.openSync(file, 'r')
+  try {
+    const { size } = fs.fstatSync(fd)
+    const tailLen = Math.min(size, 65557)
+    const tail = Buffer.alloc(tailLen)
+    fs.readSync(fd, tail, 0, tailLen, size - tailLen)
+    const eocd = tail.lastIndexOf(Buffer.from([0x50, 0x4b, 0x05, 0x06]))
+    if (eocd < 0) throw new Error('CBZ inválido')
+    const count = tail.readUInt16LE(eocd + 10)
+    const cdSize = tail.readUInt32LE(eocd + 12)
+    const cdOffset = tail.readUInt32LE(eocd + 16)
+    const cd = Buffer.alloc(cdSize)
+    fs.readSync(fd, cd, 0, cdSize, cdOffset)
+    const entries = []
+    const lh = Buffer.alloc(30)
+    for (let p = 0, i = 0; i < count; i++) {
+      const method = cd.readUInt16LE(p + 10)
+      const compSize = cd.readUInt32LE(p + 20)
+      const nameLen = cd.readUInt16LE(p + 28)
+      const extraLen = cd.readUInt16LE(p + 30)
+      const commentLen = cd.readUInt16LE(p + 32)
+      const localOffset = cd.readUInt32LE(p + 42)
+      const name = cd.toString('utf8', p + 46, p + 46 + nameLen)
+      p += 46 + nameLen + extraLen + commentLen
+      if (!IMAGE_EXT.test(name) || name.startsWith('__MACOSX')) continue
+      fs.readSync(fd, lh, 0, 30, localOffset)
+      const start = localOffset + 30 + lh.readUInt16LE(26) + lh.readUInt16LE(28)
+      entries.push({ name, method, start, size: compSize })
+    }
+    return entries.sort((a, b) => a.name.localeCompare(b.name, undefined, { numeric: true }))
+  } finally {
+    fs.closeSync(fd)
+  }
+}
+
+export function imageType(name) {
+  const ext = name.split('.').pop().toLowerCase()
+  return { jpg: 'image/jpeg', jpeg: 'image/jpeg', png: 'image/png', webp: 'image/webp', gif: 'image/gif', avif: 'image/avif' }[ext] || 'application/octet-stream'
+}
+
+// Envía una entrada del CBZ por la respuesta HTTP.
+export function sendCbzEntry(res, file, entry) {
+  const headers = { 'Content-Type': imageType(entry.name), 'Cache-Control': 'private, max-age=604800' }
+  if (entry.method === 0) {
+    res.writeHead(200, { ...headers, 'Content-Length': entry.size })
+    fs.createReadStream(file, { start: entry.start, end: entry.start + entry.size - 1 }).pipe(res)
+    return
+  }
+  if (entry.method !== 8) throw new Error(`Compresión ZIP no soportada (${entry.method})`)
+  const fd = fs.openSync(file, 'r')
+  const buf = Buffer.alloc(entry.size)
+  try { fs.readSync(fd, buf, 0, entry.size, entry.start) } finally { fs.closeSync(fd) }
+  res.writeHead(200, headers)
+  res.end(zlib.inflateRawSync(buf))
+}