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/services/appService.ts

@@ -77,7 +77,7 @@ export abstract class BaseAppService implements AppService {
   abstract selectFiles(name: string, extensions: string[]): Promise<string[]>;
   abstract saveFile(
     filename: string,
-    content: string | ArrayBuffer,
+    content: string | ArrayBuffer | null,
     options?: {
       filePath?: string;
       mimeType?: string;