Add quick-add mode for categories

N (from anywhere) or n (in Category panel) opens an inline prompt
to add categories one after another without typing :add-cat each time.

- Yellow border + prompt distinguishes it from item-add (green)
- Enter / Tab adds the category and clears the buffer, staying open
- Esc returns to the category list
- Cursor automatically moves to the newly added category

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/help.rs

@@ -51,7 +51,8 @@ impl Widget for HelpWidget {
             ("", "", norm),
             ("Panels", "", head),
             ("  F", "Toggle Formula panel  (n:new  d:del)", key),
-            ("  C", "Toggle Category panel  (Space:cycle-axis)", key),
+            ("  C", "Toggle Category panel  (n:new-cat  a:add-items)", key),
+            ("  N", "New category quick-add (from anywhere)", key),
             ("  V", "Toggle View panel  (n:new  d:del  Enter:switch)", key),
             ("  Tab", "Focus next open panel", key),
             ("", "", norm),