Fix op page UI: button underline, tab panel duplication, header spacing

- /op/dashboard: remove underline from <a class="secondaryButton"> by adding
  text-decoration:none + inline-flex centering on .primaryButton/.secondaryButton/
  .dangerButton, plus margin-left:auto on .dashboardActions so the buttons stick
  to the right side of the row even when the header is laid out as flex+wrap.
- /op/list/<pack>: fix the duplicate "save/clear + playlist URL" UI showing in
  the active tab. .tabPanel had a baseline `display: block;` that overrode the
  browser default `[hidden] { display: none }`. Add an explicit
  `.tabPanel[hidden] { display: none !important }` rule so the inactive panel
  actually hides.
- /op/list, /op/list/<pack>, /op/datapack: bump the gap between the "돌아가기"
  ghost link and the page title from 8px to 20px.
- docs/yt-dlp-setup.md: install guide (single-binary curl + pipx + apt) since
  the server doesn't have pip/pipx/yt-dlp installed. Server keeps the graceful
  "수동 입력으로 진행" fallback when yt-dlp is missing.

views/op/datapack.ejs

@@ -13,7 +13,7 @@
     <section class="dashboardHeader">
       <div>
         <a class="ghostLink" href="/op/dashboard">← 돌아가기</a>
-        <h1 style="margin-top:8px;">데이터팩 수정</h1>
+        <h1 style="margin-top:20px;">데이터팩 수정</h1>
       </div>
     </section>