feat: add sidebar enhancement components (Tier 7)

New components:
- KeyboardShortcutsPanel: Collapsible panel for displaying keyboard shortcuts
  with support for categories and compact mode
- SidebarSection: Grouped navigation section with optional collapse behavior

New types:
- KeyboardShortcut: Interface for shortcut definitions (keys, label, category)

These components enable building feature-rich sidebars like Memoro's while
keeping the shared-ui Sidebar flexible and data-driven.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

packages/shared-ui/src/molecules/index.ts

@@ -24,3 +24,4 @@ export { EmptyState } from './feedback';
 export { default as ModalFooter } from './ModalFooter.svelte';
 export { default as DataCard } from './DataCard.svelte';
 export { default as PageHeader } from './PageHeader.svelte';
+export { default as KeyboardShortcutsPanel } from './KeyboardShortcutsPanel.svelte';