feat: add Inbox sidebar section showing unlinked notes

Inbox shows notes without valid outgoing relationships (body wikilinks
or frontmatter refs), helping users find captured but unorganized notes.
Includes time-period filter pills (This week/month/quarter/All time),
Cmd+K command, and macOS menu bar entry. Broken wikilinks (targeting
non-existent notes) are not counted as relationships.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useMenuEvents.ts

@@ -75,6 +75,7 @@ const FILTER_MAP: Record<string, SidebarFilter> = {
   'go-archived': 'archived',
   'go-trash': 'trash',
   'go-changes': 'changes',
+  'go-inbox': 'inbox',
 }
 
 type OptionalHandler =