feat: configurable OPDS download folder and filename format (#2571)

src/util/OpdsFilename.h

@@ -0,0 +1,18 @@
+#pragma once
+#include <cstdint>
+#include <string>
+
+// On-disk filename format for books downloaded from an OPDS server. Stored as a
+// uint8_t in CrossPointSettings; cast to this enum at the call sites. `Count` is
+// the number of selectable formats (used to cycle the setting in the UI).
+enum class OpdsFilenameFormat : uint8_t {
+  AuthorTitle = 0,  // "Author - Title.epub" (default; matches legacy behaviour)
+  TitleAuthor = 1,  // "Title - Author.epub"
+  TitleOnly = 2,    // "Title.epub"
+  Count = 3,
+};
+
+// Composes and sanitizes the on-disk filename (including the ".epub" extension)
+// for a downloaded OPDS book, according to `format`. When the author is empty,
+// every format collapses to just the sanitized title. Pure: no I/O, no globals.
+std::string opdsBookFilename(const std::string& author, const std::string& title, OpdsFilenameFormat format);