feat: implement video site per README spec

- Express + EJS + express-session stack (auth/navbar ported from minecraft_launcher)
- Public: main folder list, folder video grid, internal popup player (/player/:videoId)
- Admin (/op): login, folder CRUD with right-click context menu + add-folder modal
- Admin folder: video grid with right-click edit/rename/delete, "영상 추가" -> editor
- Video editor: drag-drop upload, file picker, YouTube URL probe (ETA + 5분 경고),
  background yt-dlp download with progress polling, navbar title edit, trim controls,
  save runs ffmpeg trim (original preserved)
- Filesystem storage under data/folders/<name>/<videoId>/{meta.json, original.<ext>, edited.<ext>}

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

public/player.js

@@ -0,0 +1,51 @@
+(function () {
+  var overlay = document.getElementById('playerOverlay')
+  var closeBtn = document.getElementById('playerClose')
+  var video = document.getElementById('playerVideo')
+  var titleEl = document.getElementById('playerTitle')
+
+  function openPlayer(videoId, title) {
+    // 스펙: /player/:videoId 로 이동한 것처럼 동작하면서 내부 팝업으로 띄운다.
+    // pushState 로 URL 만 바꿔, 새로고침/직접접근 시 player.ejs 가 응답한다.
+    history.pushState({ player: true, videoId: videoId }, '', '/player/' + encodeURIComponent(videoId))
+    titleEl.textContent = title || ''
+    video.src = '/api/video/' + encodeURIComponent(videoId) + '/file'
+    overlay.hidden = false
+    video.play().catch(function () { /* 자동재생 막힘 무시 */ })
+  }
+
+  function closePlayer() {
+    overlay.hidden = true
+    video.pause()
+    video.removeAttribute('src')
+    video.load()
+    // 폴더 페이지로 되돌리기
+    if (history.state && history.state.player) {
+      history.back()
+    }
+  }
+
+  document.querySelectorAll('.videoCard').forEach(function (card) {
+    card.addEventListener('click', function () {
+      var id = card.getAttribute('data-video-id')
+      var title = card.querySelector('.videoTitle')
+      openPlayer(id, title ? title.textContent : '')
+    })
+  })
+
+  if (closeBtn) closeBtn.addEventListener('click', closePlayer)
+  if (overlay) overlay.addEventListener('click', function (e) {
+    if (e.target === overlay) closePlayer()
+  })
+  window.addEventListener('popstate', function () {
+    if (!overlay.hidden) {
+      overlay.hidden = true
+      video.pause()
+      video.removeAttribute('src')
+      video.load()
+    }
+  })
+  document.addEventListener('keydown', function (e) {
+    if (e.key === 'Escape' && !overlay.hidden) closePlayer()
+  })
+})()