feat: customize which type sections are shown in sidebar

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>

src/hooks/useSectionVisibility.ts

@@ -0,0 +1,44 @@
+import { useState, useCallback } from 'react'
+
+const STORAGE_KEY = 'laputa-hidden-sections'
+
+function loadHiddenSections(): Set<string> {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY)
+    if (raw) {
+      const arr = JSON.parse(raw)
+      if (Array.isArray(arr)) return new Set(arr)
+    }
+  } catch {
+    // ignore corrupt data
+  }
+  return new Set()
+}
+
+function saveHiddenSections(hidden: Set<string>) {
+  localStorage.setItem(STORAGE_KEY, JSON.stringify([...hidden]))
+}
+
+export function useSectionVisibility() {
+  const [hiddenSections, setHiddenSections] = useState<Set<string>>(loadHiddenSections)
+
+  const toggleSection = useCallback((type: string) => {
+    setHiddenSections((prev) => {
+      const next = new Set(prev)
+      if (next.has(type)) {
+        next.delete(type)
+      } else {
+        next.add(type)
+      }
+      saveHiddenSections(next)
+      return next
+    })
+  }, [])
+
+  const isSectionVisible = useCallback(
+    (type: string) => !hiddenSections.has(type),
+    [hiddenSections],
+  )
+
+  return { hiddenSections, toggleSection, isSectionVisible }
+}