fix: page through all chapters when the API omits total

listAllChapters used `page.total ?? all.length` as the loop bound. The
/chapters endpoint always returns `total: null`, so a full 200-row first
page broke the loop immediately — the reader dead-ended past chapter 200.
Stop on a short page instead; only trust `total` when actually reported.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.128.3"
+version = "0.128.4"
 dependencies = [
  "anyhow",
  "argon2",