Restore the novel script's lost module-scope constants (#74) (#75)

> *This was generated by AI during triage.*

Fixes #74.

`novel-bookmark.user.js` was split out of `manga-bookmark.user.js` and lost four module-scope constants. Every use of them is behind a `try/catch` or a fire-and-forget promise, so the `ReferenceError`s were swallowed rather than reported.

| constant | used at | effect while missing |
| --- | --- | --- |
| `LATEST_CHECK_THROTTLE_MS` | `:722` | `backgroundRefreshLatest()` throws before computing `due` — no background latest-check ever runs for novels (the symptom in #74) |
| `LATEST_CHECK_BATCH` | `:724` | same throw |
| `CACHE_KEY` | `:215`, `:224` | `loadCache()` always returns `[]`, `saveCache()` silently no-ops — the local cache never persists |
| `LASTCHECKED_KEY` | `:232`, `:241` | last-checked map never persists, so the throttle would not hold even once the first two are defined |

#74 named only the two throttle constants. The two cache keys are the same lost lines with the same root cause, so they are restored here too — fixing only the pair the issue named would leave `backgroundRefreshLatest()` re-fetching every series on every navigation, because `saveLastChecked()` would still be a no-op.

Values and comments copied verbatim from `manga-bookmark.user.js:37-43`; throttle 4h, batch 1.

## Verification

- `node --check userscript/novel-bookmark.user.js` — clean.
- `node --test userscript/test/logic.test.js userscript/test/novel-logic.test.js` — 47/47 pass.
- New test `every SCREAMING_CASE constant the script uses is declared in it` scans both scripts (comments and string literals stripped first, so prose and SVG path data do not trip it). Confirmed it fails — 1 failing test — when `LATEST_CHECK_BATCH` is deleted again, and passes when restored.

A behavioural test cannot reach this: the storage helpers and the background refresh are exactly the layers the harness does not cover (see the `testing-the-userscript` skill), and the errors are swallowed anyway. A static guard is the only instrument that sees this bug class.

On-device confirmation that the ember now lights for novels is still outstanding — that needs Violentmonkey against a live novelfull/lightnovelworld page.

Reviewed-on: #75
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

userscript/novel-bookmark.user.js

@@ -25,6 +25,13 @@
   // This script owns the novel library; the manga script is a separate install
   // with its own prefix, so the two never share a cache, a queue or a panel.
   const STORE_PREFIX = "bmgr:novel:";
+  const CACHE_KEY = STORE_PREFIX + "cache";
+
+  // Per-device record of when each series was last checked for new chapters.
+  // Deliberately not synced: each device does its own checking.
+  const LASTCHECKED_KEY = STORE_PREFIX + "lastchecked";
+  const LATEST_CHECK_THROTTLE_MS = 4 * 60 * 60 * 1000;
+  const LATEST_CHECK_BATCH = 1; // series fetched per navigation // series fetched per navigation
 
   // Which library this script's rows belong to. The manga script is a separate
   // install that declares "manga"; the backend keeps whichever it is told.