fix(settings): case sync asks and merges only while it is on

- session-ui.js: no GET /api/settings on a tab return unless this device's cached syncCaseState is true
- system-routes.ts: caseSettings is merged only while syncCaseState is true (as stored or set by the same PUT)
- sync-case-state tests cover both; wiki notes the stored options stay when sync is turned off

src/web/public/session-ui.js

@@ -3759,6 +3759,9 @@ Object.assign(CodemanApp.prototype, {
   /** On returning to the tab: take the other devices' case options and selected case, if the server has sync on. */
   async _syncCaseStateFromServer() {
     if (this._settingsReadOnly?.()) return;
+    // Off on this device: no request on every tab return. Turning it on elsewhere is learned from the
+    // next settings load; turning it off elsewhere still reaches a device whose cached switch is on.
+    if (this.loadAppSettingsFromStorage().syncCaseState !== true) return;
     const seq = (this._caseSyncRefresh = (this._caseSyncRefresh || 0) + 1);
     try {
       await this._caseSyncChain; // this device's own saves land first, so the copy read next includes them

src/web/routes/system-routes.ts

@@ -1145,10 +1145,14 @@ export function registerSystemRoutes(
       if (settings.tunnelEnabled === true && !tunnelEnableAuthorized) delete settingsToStore.tunnelEnabled;
       // caseSettings merges per case inside the serialised writer, so two devices saving different cases both land.
       const { caseSettings: incomingCases, ...plainSettings } = settingsToStore;
+      // The map is read and written only while syncCaseState is on, so a PUT that carries it with the setting off
+      // (as stored, or as this same PUT sets it) leaves the stored map alone.
       const { before: existing, after } = await mutateSettingsFile((current) => ({
         ...current,
         ...plainSettings,
-        ...(incomingCases ? { caseSettings: mergeCaseSettings(current.caseSettings, incomingCases as never) } : {}),
+        ...(incomingCases && (plainSettings.syncCaseState ?? current.syncCaseState) === true
+          ? { caseSettings: mergeCaseSettings(current.caseSettings, incomingCases as never) }
+          : {}),
       }));
       const merged = after ?? { ...existing, ...plainSettings };