refactor(share): make saveFile content nullable for path-based shares (#4424)

The book "Send" flow had to pass `new ArrayBuffer(0)` to saveFile purely to
satisfy the type-checker: the content arg is ignored on the native share
path when `options.filePath` points at an already-on-disk file. Widen
saveFile's content parameter to `string | ArrayBuffer | null` across the
AppService contract so callers can hand off a file by path without buffering
it into memory, and pass `null` from the Send flow instead of a throwaway
empty buffer.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/readest-app/src/app/library/components/Bookshelf.tsx

@@ -517,10 +517,9 @@ const Bookshelf: React.FC<BookshelfProps> = ({
       // whole epub/pdf into memory just to have saveFile write it back
       // to disk.
       const absoluteFilePath = await appService.resolveFilePath(path, base);
-      // saveFile's binary branch wants an ArrayBuffer; with `filePath`
-      // set the content arg is ignored on the native share path, but
-      // we still pass an empty buffer so the type-checker is happy.
-      await appService.saveFile(shareFilename, new ArrayBuffer(0), {
+      // `null` content: there's nothing to write — the file already lives at
+      // `filePath`, which the native share path reads directly.
+      await appService.saveFile(shareFilename, null, {
         share: true,
         mimeType,
         filePath: absoluteFilePath,