Add Fetch Disks button and Anleitung copy-protection viewer

- Cross-compile nibtools for Android NDK; download all 8 episode disks
  from the Internet Archive C64 Preservation Project as NBZ and convert
  to G64 via JNI (throwaway-pthread to contain nibconv's exit() calls)
- Bundle manual transcription as APK asset; 📖 button opens scrollable
  viewer with Seite→Absatz→Zeile→Wort lookup hierarchy documented

CLAUDE.md

@@ -118,7 +118,8 @@ during development, but nothing in the Gradle build copies them into the APK.
 | `C64DisplayView.kt` | SurfaceView — blits 320×200 ARGB framebuffer, scaled with correct aspect ratio |
 | `C64KeyboardView.kt` | Multi-touch virtual C64 keyboard; fires `KeyEventListener` on press/release |
 | `jni/vice_jni.c` | JNI wrapper — custom VICE video canvas writes frames to `g_framebuf[320×200]` |
-| `jni/CMakeLists.txt` | NDK build; conditionally links `libvice.a` when `HAVE_VICE_SRC=1` |
+| `jni/nibconv_jni.c` | JNI wrapper around nibtools' nibconv — converts downloaded NIB/NBZ dumps to G64 |
+| `jni/CMakeLists.txt` | NDK build; conditionally links `libvice.a`/`libnibtools.a` when their `HAVE_*` flags are set |
 
 ### Disk images
 
@@ -127,3 +128,34 @@ same external files directory as the ROMs (via USB or adb), then load via:
 ```kotlin
 engine.loadDisk(getExternalFilesDir(null)!!.absolutePath + "/schwert_und_magie_1.d64")
 ```
+
+Alternatively, the in-app **Download Disks** button (next to Import Disks) fetches all
+8 episode disks from the Internet Archive's C64 Preservation Project and converts them
+to G64 automatically — see "Disk download (nibtools)" below.
+
+### Disk download (nibtools)
+
+The Internet Archive's C64 Preservation Project only has these disks as `.nbz`
+(nibtools' compressed raw-GCR NIB format) — not `.d64`. `MainActivity.downloadDisks()`
+fetches each of the 8 `.nbz` files (one per disk side) and runs them through nibtools'
+`nibconv` to produce G64 images (VICE attaches G64 exactly like D64, detecting the
+format from file content, not the extension). G64 was chosen over the lossy D64
+reconstruction nibconv also supports, since these old dumps aren't always cleanly
+sector-readable — G64 preserves whatever the original drive actually saw.
+
+Only `nibconv`'s pure file-format conversion code is built (`gcr.c prot.c fileio.c
+crc.c md5.c lz.c nibconv.c`) — nibtools' hardware-access tools (`nibread`/`nibwrite`,
+which talk to a real 1541 over OpenCBM) are not needed and not built. `jni/nibtools_android/`
+stubs out the OpenCBM header so the unused declarations that pull it in still compile.
+
+**nibtools compilation is automatic**, the same way as VICE:
+1. The Gradle `buildNibtools` task runs before every native CMake build
+2. It calls `app/src/main/jni/build_nibtools.sh`, which unpacks `res/nibtools-<rev>.tar.gz`
+   and cross-compiles the files above into `nibtools-libs/<abi>/libnibtools.a`
+3. `CMakeLists.txt` auto-detects the library via `EXISTS` and links it in
+
+nibconv's `main()` is renamed to `nibtools_nibconv_main` at compile time (`-Dmain=...`)
+so it can coexist in the same shared library as the JNI entry points. It's invoked
+from `nibconv_jni.c` on a throwaway pthread — nibconv calls `exit()` on malformed input,
+which is wrapped (`-Wl,--wrap=exit`) to `pthread_exit()` so a bad conversion only kills
+that disposable thread, never the app process or the calling JNI thread.