(core) Add viewport meta tag conditionally, and show a toggle for it on small devices.

Summary:
- Enable narrow-screen layout for home page
- Clean up margins/spacing on small-screen home page
- Use "<768" as small-screen condition rather than "<=768".
- Include meta-viewport tag conditionally, off by default.
- Include "Toggle Mobile Mode" option in AccountMenu to toggle it on.
- In a test, add an after() clause to restore window size even when test fails

Test Plan: Only tested manually on iPhone (Safari & FF).

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: cyprien

Differential Revision: https://phab.getgrist.com/D2708

app/client/ui/AppUI.ts

@@ -100,6 +100,7 @@ function pagePanelsHome(owner: IDisposableOwner, appModel: AppModel) {
     },
     headerMain: createTopBarHome(appModel),
     contentMain: createDocMenu(pageModel),
+    optimizeNarrowScreen: true,
   });
 }