Inline edit, URL-driven meta refresh, drag gap animation, copy-from-music

Music list tab:
- Title + artist are now contenteditable in-place. Typing updates state on
  every input event; Enter blurs (no embedded newlines), and on Save the
  current DOM text is re-synced into state.music[i].title/.artist before
  the JSON POST so a quick-save right after typing doesn't drop the last
  keystroke. While focused, the field is highlighted with the accent
  outline and unclamped so long titles wrap. Empty cells show a muted
  placeholder via :empty::before { content: attr(data-placeholder) }.
- Edit modal "save" now POSTs the new URL to /op/list/<pack>/video-meta
  (new endpoint backed by yt-dlp --dump-json --no-playlist) and patches
  state.music[i] with the returned title/channel/durationSec. If yt-dlp
  is unavailable or the lookup fails, the modal asks the user whether to
  apply just the URL change without metadata.
- Drag-and-drop UX: replaced the simple "highlight target" feedback with a
  "gap opens above the target" animation. The hovered row gets a
  .dropAbove class that animates margin-top to 64px via CSS transition,
  visually carving out the slot the dragged item will land in. Insertion
  math is now strictly "drop before the hover target" (with the
  srcIdx<dstIdx → dstIdx-1 adjustment after the splice removal), matching
  what the gap implies. Contenteditable spans no longer hijack drag start
  — on focus the parent <li>.draggable is flipped off so the user can
  freely select text, and back on at blur.

Image list tab:
- New "음악목록에서 가져오기" button. Copies state.music[*].url into
  state.images, which (via the existing thumbUrl() helper) renders each
  song's YouTube thumbnail as the image. Behind a confirm prompt because
  it replaces the entire image list.
- Same drag gap animation (.dropAbove → margin-left 80px) applied to the
  grid cards for consistency.

Server:
- youtube.ts: add fetchVideoMeta(url) using the same ensureYtDlp() path
  (auto-installed binary under %appdata%/.mc_custom). Returns one
  YtPlaylistEntry or null.
- routes/op.ts: POST /op/list/:packName/video-meta. 400 on missing URL,
  503 NO_YTDLP if the auto-install failed, 500 on other yt-dlp errors,
  200 { ok: true, entry } otherwise.

Smoke test (Rick Astley URL) returns
title=Rick Astley - Never Gonna..., channel=Rick Astley, durationSec=213.

src/server/routes/op.ts

@@ -13,7 +13,7 @@ import {
   savePackList
 } from '../../shared/store'
 import { fetchReleaseVersions } from '../../shared/mojang'
-import { fetchPlaylistEntries, YtDlpUnavailableError } from '../youtube'
+import { fetchPlaylistEntries, fetchVideoMeta, YtDlpUnavailableError } from '../youtube'
 import { requireAuth } from '../middleware/auth'
 import type { PackDefinition, PackList } from '../../shared/types'
 
@@ -174,6 +174,30 @@ opRouter.post('/op/list/:packName', requireAuth, async (req, res, next) => {
   }
 })
 
+// 단일 영상 메타데이터 조회 (음악 항목 수정에서 URL 변경 시 자동 갱신용).
+// body: { url: string }
+opRouter.post('/op/list/:packName/video-meta', requireAuth, async (req, res) => {
+  const url = pickFirstValue(req.body?.url).trim()
+  if (!url) {
+    res.status(400).json({ ok: false, message: '영상 주소를 입력해 주세요.' })
+    return
+  }
+  try {
+    const entry = await fetchVideoMeta(url)
+    if (!entry) {
+      res.status(404).json({ ok: false, message: '메타데이터를 찾을 수 없습니다.' })
+      return
+    }
+    res.json({ ok: true, entry })
+  } catch (error) {
+    if (error instanceof YtDlpUnavailableError) {
+      res.status(503).json({ ok: false, message: error.message, code: 'NO_YTDLP' })
+      return
+    }
+    res.status(500).json({ ok: false, message: (error as Error).message })
+  }
+})
+
 // 플레이리스트 주소를 yt-dlp 로 풀어 목록 후보를 반환.
 // body: { url: string }
 opRouter.post('/op/list/:packName/playlist', requireAuth, async (req, res) => {