Rename app to EasyView and make APK build cross-platform

- App display name -> EasyView (Android label, PWA manifest, page titles,
  about/guide text, APK artifact name)
- scripts/build-apk.mjs: auto-detect JDK/SDK/Gradle toolchain instead of
  hardcoded Windows paths; pick highest build-tools; platform 34 fallback;
  path.delimiter classpaths; npm run build without cmd.exe on POSIX
- Add build-apk.sh one-shot Linux build wrapper (loads ven env + keystore)
- Signing keystore renamed to easyview.jks (alias easyview),
  env vars EASYVIEW_KEYSTORE_PASS / EASYVIEW_KEY_PASS

src/doc/docViewer.ts

@@ -315,7 +315,7 @@ export class DocViewer {
     add('rotate', 'rotate-cw', '旋转 90°', () => this.rotatePage())
     add('invert', 'moon', '反色阅读', () => this.toggleInvert())
     this.fullscreenBtn = add('fullscreen', 'maximize', '全屏', () => void this.toggleFullscreen())
-    add('about', 'info', '关于万能看图', () => showAbout())
+    add('about', 'info', '关于 EasyView', () => showAbout())
     return menu
   }