fix(dashboard): improve task filter wording to distinguish "No Task" from real tasks (#502)

Previously, the filter text showed "filtered by 1 task" when only "No Task"
was selected, which was confusing because it implied there was 1 real task.

Now the filter text clearly distinguishes:
- "unassigned only" - when only "No Task" is selected
- "filtered by X tasks" - when only real tasks are selected
- "filtered by X tasks + unassigned" - when both are selected

Before:
  (filtered by 1 task)  ← confusing, looks like there's a real task

After:
  (unassigned only)  ← clear, shows messages without task

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mbt1909432

dashboard/app/project/[id]/session/[sessionId]/messages/messages-page-client.tsx

@@ -731,12 +731,26 @@ export function MessagesPageClient({
                 )}
               </>
             )}
-            {selectedTaskIds.size > 0 && (
-              <span className="text-xs text-muted-foreground ml-2">
-                Showing {filteredTimelineItems.filter(i => i.kind === 'message').length} of {allMessages.length} messages
-                (filtered by {selectedTaskIds.size} {selectedTaskIds.size === 1 ? 'task' : 'tasks'})
-              </span>
-            )}
+            {selectedTaskIds.size > 0 && (() => {
+              const hasNoTask = selectedTaskIds.has(NO_TASK_SENTINEL);
+              const realTaskCount = selectedTaskIds.size - (hasNoTask ? 1 : 0);
+
+              let filterText = "";
+              if (hasNoTask && realTaskCount > 0) {
+                filterText = ` (filtered by ${realTaskCount} ${realTaskCount === 1 ? 'task' : 'tasks'} + unassigned)`;
+              } else if (hasNoTask) {
+                filterText = " (unassigned only)";
+              } else {
+                filterText = ` (filtered by ${realTaskCount} ${realTaskCount === 1 ? 'task' : 'tasks'})`;
+              }
+
+              return (
+                <span className="text-xs text-muted-foreground ml-2">
+                  Showing {filteredTimelineItems.filter(i => i.kind === 'message').length} of {allMessages.length} messages
+                  {filterText}
+                </span>
+              );
+            })()}
           </div>
         )}
       </div>