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

test/sync-case-state-server.test.ts

@@ -64,6 +64,17 @@ describe('shared case options', () => {
     expect(readSettings().caseSettings).toEqual({ alpha: on, beta: off, gamma: on });
   });
 
+  it('ignores a caseSettings PUT while syncCaseState is off, and takes one that turns it on', async () => {
+    seed({ syncCaseState: false, caseSettings: { kept: off } });
+    expect((await send('PUT', '/api/settings', { caseSettings: { alpha: on } })).status).toBe(200);
+    expect(readSettings().caseSettings).toEqual({ kept: off });
+    seed({});
+    await send('PUT', '/api/settings', { caseSettings: { alpha: on } });
+    expect(readSettings().caseSettings).toBeUndefined();
+    await send('PUT', '/api/settings', { syncCaseState: true, caseSettings: { alpha: on } });
+    expect(readSettings().caseSettings).toEqual({ alpha: on });
+  });
+
   it('keeps the other cases when one is saved again, and every other setting', async () => {
     seed({ syncCaseState: true, lastUsedCase: 'x', caseSettings: { alpha: on, beta: off } });
     await send('PUT', '/api/settings', { caseSettings: { alpha: off } });

test/sync-case-state.test.ts

@@ -188,11 +188,11 @@ describe('returning to the tab', () => {
     expect(app._caseStateSyncOn()).toBe(false);
   });
 
-  it('starts following when another device turned sync on', async () => {
-    const { app, select, settings } = makeApp({}, () => env({ syncCaseState: true, lastUsedCase: 'beta' }));
+  it('asks for nothing on a tab return while this device has sync off', async () => {
+    const { app, select, calls } = makeApp({}, () => env({ syncCaseState: true, lastUsedCase: 'beta' }));
     await app._syncCaseStateFromServer();
-    expect(settings.syncCaseState).toBe(true);
-    expect(select.value).toBe('beta');
+    expect(calls).toEqual([]);
+    expect(select.value).toBe('alpha');
   });
 
   it('does nothing for a read-only identity', async () => {