feat(libraries): add pin-to-top for media library list (#10)

Allow users to pin frequently used libraries to the top of the
Libraries page. Pinned order is stored in localStorage and applied
to both entry cards and preview shelves.

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: truewhile <truewhile@users.noreply.github.com>

web/src/utils/pinnedLibraries.ts

@@ -0,0 +1,48 @@
+const STORAGE_KEY = 'mmtl_pinned_libraries'
+
+function parsePinnedIds(raw: string | null): string[] {
+  if (!raw) return []
+  try {
+    const parsed = JSON.parse(raw)
+    if (!Array.isArray(parsed)) return []
+    return parsed.filter((id): id is string => typeof id === 'string' && id.trim() !== '')
+  } catch {
+    return []
+  }
+}
+
+export function readPinnedLibraryIds(): string[] {
+  if (typeof window === 'undefined') return []
+  return parsePinnedIds(window.localStorage.getItem(STORAGE_KEY))
+}
+
+export function writePinnedLibraryIds(ids: string[]): void {
+  if (typeof window === 'undefined') return
+  window.localStorage.setItem(STORAGE_KEY, JSON.stringify(ids))
+}
+
+export function togglePinnedLibraryId(id: string): string[] {
+  const current = readPinnedLibraryIds()
+  const index = current.indexOf(id)
+  const next = index >= 0 ? current.filter((item) => item !== id) : [...current, id]
+  writePinnedLibraryIds(next)
+  return next
+}
+
+export function isLibraryPinned(id: string, pinnedIds: string[]): boolean {
+  return pinnedIds.includes(id)
+}
+
+export function sortLibraryPreviews<T extends { library: { id: string } }>(items: T[], pinnedIds: string[]): T[] {
+  if (pinnedIds.length === 0) return items
+  const rank = new Map(pinnedIds.map((id, index) => [id, index]))
+  return [...items].sort((a, b) => {
+    const aRank = rank.get(a.library.id)
+    const bRank = rank.get(b.library.id)
+    const aPinned = aRank !== undefined
+    const bPinned = bRank !== undefined
+    if (aPinned !== bPinned) return aPinned ? -1 : 1
+    if (aPinned && bPinned) return aRank - bRank
+    return 0
+  })
+}