Add quick multi-item entry to categories

Two new ways to add multiple items without repeating yourself:

1. :add-items <category> item1 item2 item3 ...
   Adds all space-separated items in one command.

2. Category panel quick-add mode (press 'a' or 'o' on a category):
   - Opens an inline prompt at the bottom of the panel
   - Enter adds the item and clears the buffer — stays open for next entry
   - Tab does the same as Enter
   - Esc closes and returns to the category list
   - The panel border turns green and the title updates to signal add mode
   - Item count in the category list updates live as items are added

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

src/ui/help.rs

@@ -67,7 +67,8 @@ impl Widget for HelpWidget {
             ("  :import <path.json>", "Open JSON import wizard", key),
             ("  :export [path.csv]", "Export active view to CSV", key),
             ("  :add-cat <name>", "Add a category", key),
-            ("  :add-item <cat> <item>", "Add an item to a category", key),
+            ("  :add-item <cat> <item>", "Add one item to a category", key),
+            ("  :add-items <cat> a b c…", "Add multiple items at once", key),
             ("  :formula <cat> <Name=expr>", "Add a formula", key),
             ("  :add-view [name]", "Create a new view", key),
             ("", "", norm),