Add heading placeholders for h1-h6 in BlockPlaceholderPlugin

- Add placeholder text 'Heading 1' through 'Heading 6' for empty heading components
- Uses existing KEYS constants and placeholder system
- Addresses BUS-1700 ticket requirements

Co-Authored-By: nate@buster.so <nate@buster.so>

apps/web/src/components/ui/report/plugins/block-placeholder-kit.tsx

@@ -8,6 +8,12 @@ export const BlockPlaceholderKit = [
         'before:absolute before:cursor-text before:text-muted-foreground/80 before:content-[attr(placeholder)]',
       placeholders: {
         [KEYS.p]: 'Write or type "/" for commands...',
+        [KEYS.h1]: 'Heading 1',
+        [KEYS.h2]: 'Heading 2',
+        [KEYS.h3]: 'Heading 3',
+        [KEYS.h4]: 'Heading 4',
+        [KEYS.h5]: 'Heading 5',
+        [KEYS.h6]: 'Heading 6',
       },
       query: ({ path }) => {
         return path.length === 1;