Adds a "Customize sections" button above section groups in the sidebar.
Clicking it opens a popover with per-type toggle switches. Hidden sections
disappear from the sidebar entirely but notes remain accessible via
All Notes and search. Visibility state is persisted in localStorage.
- New useSectionVisibility hook: manages hidden sections set in localStorage
- Sidebar: "Customize sections" button + popover with toggle UI
- Outside-click closes the popover
- allSectionGroups (built-in + custom) filtered by visibility before render
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
