feat(ui): admin Hash Conflicts page and nav entry

New /admin/hash-conflicts page (admin-only) listing pending
content-duplicate groups. Each group card shows the library, a
shortened SHA-256, and one row per copy with title, author, path,
size, and per-copy reading-data counts (progress, highlights,
bookmarks, notes, collections) - copies that own user data are
highlighted so the keep choice is informed.

Per copy: 'Keep this copy' merges the other copies' child rows into
it and deletes them. Per group: 'Keep both' for intentional
duplicates. Both confirm first, resolve via htmx POST, and swap the
card for a resolved confirmation inline. The confirmation fragment is
built inline in the handler rather than the templates package
(templates imports handlers; a back-import would be a cycle).

Empty state shown when no conflicts are pending. Adds a 'Hash
Conflicts' entry to the admin sidebar section between Libraries and
Users.

templates/header.templ

@@ -159,6 +159,10 @@ templ Header(user User, currentPath string) {
 							@Icon("library", "h-5 w-5 shrink-0")
 							<span>Libraries</span>
 						</a>
+						<a href="/admin/hash-conflicts" class={ activeClass(currentPath, "/admin/hash-conflicts") }>
+							@Icon("copy", "h-5 w-5 shrink-0")
+							<span>Hash Conflicts</span>
+						</a>
 						<a href="/admin/users" class={ activeClass(currentPath, "/admin/users") }>
 							@Icon("users", "h-5 w-5 shrink-0")
 							<span>Users</span>