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.

public/styles.css

@@ -171,9 +171,16 @@ body.siteBody.centerLayout {
   gap: 12px;
 }
 
+.dashboardHeader > div { min-width: 0; }
 .dashboardHeader h1 { margin: 0; font-size: 24px; }
 
-.dashboardActions { display: flex; gap: 8px; }
+.dashboardActions {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+  margin-left: auto;
+  align-items: center;
+}
 
 .inlineForm { margin: 0; }
 
@@ -193,6 +200,10 @@ body.siteBody.centerLayout {
   cursor: pointer;
   font-size: 14px;
   font-weight: 600;
+  text-decoration: none;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
 }
 
 .primaryButton:hover { background: var(--accent-hover); }
@@ -205,6 +216,10 @@ body.siteBody.centerLayout {
   border-radius: 8px;
   cursor: pointer;
   font-size: 14px;
+  text-decoration: none;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
 }
 
 .secondaryButton:hover { border-color: var(--accent); }
@@ -218,6 +233,10 @@ body.siteBody.centerLayout {
   cursor: pointer;
   font-size: 14px;
   font-weight: 600;
+  text-decoration: none;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
 }
 
 .dangerButton:hover { background: #d73a48; }
@@ -370,6 +389,7 @@ body.siteBody.centerLayout {
 .tabBtn.active { color: var(--text); border-bottom-color: var(--accent); }
 
 .tabPanel { display: block; }
+.tabPanel[hidden] { display: none !important; }
 
 .listActionsRow { display: flex; gap: 8px; align-items: center; margin-bottom: 12px; flex-wrap: wrap; }
 .statusText { font-size: 13px; color: var(--text-muted); margin-left: 8px; }