docs: add missing changelog entry for theming reliability (R568) (#603)

* R568: add missing changelog entry for schema migration hardening

* R568: remove ticket references from changelog entry

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Co-authored-by: ryacub <ryacub@users.noreply.github.com>

Author: ryacub

CHANGELOG.md

@@ -64,6 +64,7 @@ The format is a modified version of [Keep a Changelog](https://keepachangelog.co
 - Remove unused `CoroutineScope` parameter from `PlayerMpvInitializer` constructor
 - Remove dead `rollbackToLastGood()` stub and `ROLLBACK_NOT_AVAILABLE` error code from `LightNovelPluginManager`; converted 4 deferred TODO comments to tracked GitHub issues (#536–#539)
 - Backfilled missing `Unreleased` entries from previously merged Codex PRs, deduped against existing changelog items
+- Harden custom accent preference schema migration safety: schema-first read, malformed/unknown-version fallback to legacy/unset, dual-write compatibility for downgrade paths, and delete/isSet reactive stability improvements
 
 ## [0.18.1.75] - 2026-03-13