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;

apps/readest-app/src/services/nativeAppService.ts

@@ -633,7 +633,7 @@ export class NativeAppService extends BaseAppService {
 
   async saveFile(
     filename: string,
-    content: string | ArrayBuffer,
+    content: string | ArrayBuffer | null,
     options?: {
       filePath?: string;
       mimeType?: string;
@@ -655,7 +655,7 @@ export class NativeAppService extends BaseAppService {
           shareablePath = await this.resolveFilePath(filename, 'Temp');
           if (typeof content === 'string') {
             await writeTextFile(shareablePath, content);
-          } else {
+          } else if (content) {
             await writeFile(shareablePath, new Uint8Array(content));
           }
         }
@@ -688,7 +688,7 @@ export class NativeAppService extends BaseAppService {
 
       if (typeof content === 'string') {
         await writeTextFile(filePath, content);
-      } else {
+      } else if (content) {
         await writeFile(filePath, new Uint8Array(content));
       }
       return true;

apps/readest-app/src/services/nodeAppService.ts

@@ -386,7 +386,7 @@ export class NodeAppService extends BaseAppService {
 
   async saveFile(
     _filename: string,
-    content: string | ArrayBuffer,
+    content: string | ArrayBuffer | null,
     options?: {
       filePath?: string;
       mimeType?: string;
@@ -399,7 +399,7 @@ export class NodeAppService extends BaseAppService {
       await fsp.mkdir(nodePath.dirname(filepath), { recursive: true });
       if (typeof content === 'string') {
         await fsp.writeFile(filepath, content, 'utf-8');
-      } else {
+      } else if (content) {
         await fsp.writeFile(filepath, Buffer.from(content));
       }
       return true;

apps/readest-app/src/services/webAppService.ts

@@ -333,7 +333,7 @@ export class WebAppService extends BaseAppService {
 
   async saveFile(
     filename: string,
-    content: string | ArrayBuffer,
+    content: string | ArrayBuffer | null,
     options?: {
       filePath?: string;
       mimeType?: string;
@@ -344,6 +344,9 @@ export class WebAppService extends BaseAppService {
     },
   ): Promise<boolean> {
     const mimeType = options?.mimeType || 'application/octet-stream';
+    // Web has no filesystem path to read from, so `null` content (only the
+    // native-only "Send" flow passes it) degrades to an empty body.
+    const body = content ?? '';
     if (
       options?.share &&
       typeof navigator !== 'undefined' &&
@@ -351,7 +354,7 @@ export class WebAppService extends BaseAppService {
     ) {
       let shareData: ShareData | null = null;
       try {
-        const file = new File([content], filename, { type: mimeType });
+        const file = new File([body], filename, { type: mimeType });
         const candidate: ShareData = { files: [file], title: filename };
         if (typeof navigator.canShare !== 'function' || navigator.canShare(candidate)) {
           shareData = candidate;
@@ -377,7 +380,7 @@ export class WebAppService extends BaseAppService {
       }
     }
     try {
-      const blob = new Blob([content], { type: mimeType });
+      const blob = new Blob([body], { type: mimeType });
       const url = URL.createObjectURL(blob);
       const a = document.createElement('a');
       a.href = url;