Wave 4 (partial): live-sync harness + Library screenshots; fix authors null decode

Both wave-4 workers hit the 5h session limit ~11 minutes in and were then lost
to a sprite suspend. This commit preserves the work that landed before that,
independently verified green (assembleDebug + testDebugUnitTest, 94 tests).

F1-livesync:
- LiveSyncTest + server/live-sync-test.sh: end-to-end exercise against a real
  PocketBase (auth, push with client ids, pull, tombstones, cover round-trip).
  Gated behind LIVE_SYNC=1 so the normal test task stays green with no server.
- Fix: BookDto.authors must be nullable. PocketBase serializes an unset `json`
  field as literal null (unlike text/number, which come back ""/0), so decoding
  any real response with empty authors threw. Found by the live test; no fake
  had ever reproduced it.
- Fix: cover upload derived its media type from the filename instead of
  hardcoding image/jpeg.

F2-release:
- ScreenFixtures + LibraryScreenPaparazziTest: library populated and empty,
  light and dark (4 PNGs).

Still owed by wave 4: screenshots for the other five screens, release keystore
+ signed APK, top-level README.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016mTs3kQXQsQwonXpEq7aEw

logs/WAVE3-DONE

@@ -0,0 +1,15 @@
+=== WAVE3-DONE written 2026-09-06T10:52:02+00:00 ===
+Workers finished. The orchestrator was NOT necessarily alive for this.
+
+--- E1-shell ---
+[2026-09-06T10:46:42+00:00] E1-shell: SUCCESS after 1 attempt(s), 0 quota wait(s)
+cost=$2.8037944 turns=6
+
+--- E2-books ---
+[2026-09-06T10:24:59+00:00] E2-books: QUOTA hit (wait #1). sleeping 600s then probing again.
+[2026-09-06T10:35:41+00:00] E2-books: SUCCESS after 2 attempt(s), 1 quota wait(s)
+cost=$0.25929520000000006 turns=5
+
+NEXT: orchestrator must independently verify before accepting:
+  cd ~/bookshelf && ./tasks/gw assembleDebug && ./tasks/gw testDebugUnitTest
+  git status --porcelain   # boundary check: who touched what