fix(settings): case sync tests follow lastPickedCase, a failed pick is not undone, rollback is covered

- sync-case-state browser and stale-refresh tests use lastPickedCase (the stale test now fails without its guard)
- saveLastUsedCase runs on the case-sync chain, and the follow is skipped while this device's own pick was not stored
- _caseStateSyncOn tolerates a harness without loadAppSettingsFromStorage
- lastPickedCase added to the config-backup volatile-key test and docs; rename rollback tests cover caseSettings and lastPickedCase
- wiki row moved to System

docs/wiki/Config-Backups.md

@@ -41,7 +41,7 @@ cron jobs.
   A field a file rewrites on its own is not a change, and the comparison looks through it (the
   snapshot still stores the file exactly as written, and a restore puts those fields back as they
   were): `custom-model-hosts.json` `lastDiscoveredAt` (the model-discovery poll, every five
-  minutes), `users.json` `lastLoginAt`, `settings.json` `lastUsedCase`, `intents.json`
+  minutes), `users.json` `lastLoginAt`, `settings.json` `lastUsedCase` and `lastPickedCase`, `intents.json`
   `recentPrompts` and `updatedAt` (every captured prompt) and `docker-cases.json`
   `lastClaudeSessionId` (a Docker case's conversation id after a `/clear`) and `webviews.json`
   `lastOpenedAt` (opening a web tab). Without that, every