ShareLinks: keep player back button, disable in-item links, hide add-to-playlist, generalize guest plugin hiding

Hiding .headerBackButton for guests also removed the video player's back
button (same class), so a guest could not return from playback to the
movie page. Stop hiding that class; keep home/drawer/search hidden.

Cast, studio, genre and tag links on the detail page now have
pointer-events disabled for guests so they stay visible but inert
(clicking Studio no longer spins on a page the guest cannot load), and
the add-to-playlist/add-to-collection actions are hidden.

Add a GuestHiddenSelectors config field (comma-separated CSS selectors,
surfaced through GuestState since guests cannot read plugin config) so
other plugins injected UI can be suppressed for guest sessions without
code changes; defaults to hiding the AI Search elements.

Jellyfin.Plugin.ShareLinks/Api/ShareLinksController.cs

@@ -91,6 +91,8 @@ public sealed class ShareLinkGuestStateDto
     public DateTimeOffset? ExpiresAtUtc { get; set; }
 
     public bool LockdownEnabled { get; set; }
+
+    public string? HiddenSelectors { get; set; }
 }
 
 /// <summary>ShareLinks API surface.</summary>
@@ -291,7 +293,8 @@ public sealed class ShareLinksController : ControllerBase
                     AllowedItemId = match.ItemId,
                     ShareId = match.Id,
                     ExpiresAtUtc = match.ExpiresAtUtc,
-                    LockdownEnabled = config.GuestModeLockdownEnabled
+                    LockdownEnabled = config.GuestModeLockdownEnabled,
+                    HiddenSelectors = config.GuestHiddenSelectors
                 });
             }
         }
@@ -299,7 +302,8 @@ public sealed class ShareLinksController : ControllerBase
         return Ok(new ShareLinkGuestStateDto
         {
             IsGuest = false,
-            LockdownEnabled = config.GuestModeLockdownEnabled
+            LockdownEnabled = config.GuestModeLockdownEnabled,
+            HiddenSelectors = config.GuestHiddenSelectors
         });
     }