ShareLinks: make guest redemption actually work end to end

The sign-in bootstrap sent the auth request body as a JS object, so it
was coerced to [object Object] and AuthenticateByName returned 400. Send
JSON.stringify(...) instead.

Guest credentials were written to localStorage as a flat object under
made-up keys. jellyfin-web reads jellyfin_credentials as
{Servers:[{Id,AccessToken,UserId,...}]}, so the guest was treated as
logged out. Write that shape, pulling server Id/name from
System/Info/Public, and redirect with the 10.11 hash route
(#/details?id=...&serverId=...) instead of the legacy #!/ form that
rendered a blank page.

Creating a link now rejects folders and libraries (only movies and
episodes are shareable) so a guest cannot land on an empty tag-filtered
library. Item ids from the menu action are validated as GUIDs client
side, rejected creates are logged server side, malformed redeem tokens
return 404 instead of 500, and the admin table shows the item name and a
copyable link instead of the raw item id.

Jellyfin.Plugin.ShareLinks/Api/ShareLinksController.cs

@@ -75,6 +75,8 @@ public sealed class ShareLinkAdminRecordDto
     public int CleanupAttempts { get; set; }
 
     public string? CleanupError { get; set; }
+
+    public string? ShareUrl { get; set; }
 }
 
 /// <summary>Guest session state returned to the web client.</summary>
@@ -166,11 +168,13 @@ public sealed class ShareLinksController : ControllerBase
 
         if (request is null || string.IsNullOrWhiteSpace(request.ItemId))
         {
+            _logger.LogWarning("ShareLinks: create rejected, missing itemId.");
             return BadRequest(new { error = "Missing itemId." });
         }
 
-        if (!Guid.TryParse(request.ItemId, out var itemId))
+        if (!Guid.TryParse(request.ItemId!.Trim(), out var itemId))
         {
+            _logger.LogWarning("ShareLinks: create rejected, itemId {ItemId} is not a GUID.", request.ItemId);
             return BadRequest(new { error = "Invalid itemId." });
         }
 
@@ -189,15 +193,24 @@ public sealed class ShareLinksController : ControllerBase
         var item = _libraryManager.GetItemById(itemId);
         if (item is null)
         {
+            _logger.LogWarning("ShareLinks: create rejected, item {ItemId} not found.", itemId);
             return NotFound(new { error = "Item not found." });
         }
 
+        if (item.IsFolder)
+        {
+            _logger.LogWarning("ShareLinks: create rejected, item {ItemId} \"{ItemName}\" is a folder or library, not shareable media.", itemId, item.Name);
+            return BadRequest(new { error = "Only a movie or episode can be shared, not a folder or library. Open the title's page and try again." });
+        }
+
         try
         {
             var creatorUserId = GetCurrentUserId();
             var oneUse = request.OneUse ?? config.OneUseDefault;
             var creation = await _creationService.CreateAsync(item, creatorUserId, expiryHours, oneUse, cancellationToken).ConfigureAwait(false);
             var shareUrl = BuildShareUrl(Request, creation.RawToken);
+            creation.Record.ShareUrl = shareUrl;
+            await _store.UpdateAsync(creation.Record, cancellationToken).ConfigureAwait(false);
             return Ok(new ShareLinkCreateResponse
             {
                 ShareUrl = shareUrl,
@@ -329,7 +342,8 @@ public sealed class ShareLinksController : ControllerBase
             OneUse = record.OneUse,
             MetadataTouched = record.MetadataTouched,
             CleanupAttempts = record.CleanupAttempts,
-            CleanupError = record.CleanupError
+            CleanupError = record.CleanupError,
+            ShareUrl = record.ShareUrl
         };
     }