Give readable content its own text size (#3637)

* feat(app): separate content typography from interface size

Readable prose benefits from its own scale without inflating navigation and controls. Preserve existing users’ rendered size during migration while giving fresh desktop installs a 15px content default.

* test(app): cover independent content typography

The appearance journey now verifies that changing interface size leaves content size intact. Timeline traversal yields when a test deliberately holds the requested page, so the helper no longer waits on its own gate.

* test(desktop): cover independent content typography

The desktop appearance regression follows the renamed Interface control and verifies it does not alter Content size.

packages/app/e2e/support/helpers/timeline-pagination.ts

@@ -545,15 +545,22 @@ async function returnTimelineToSettledHistoryStart(
   page: Page,
   moveUp: () => Promise<unknown>,
 ): Promise<void> {
+  const loadingSpinner = page.getByTestId("load-older-history-spinner");
+  // A caller may only start a new history traversal after its prior page has settled.
+  await expect(loadingSpinner).toBeHidden();
   let previous = await readTimelineViewport(page);
   while (true) {
     await moveUp();
     await waitForTimelineGeometryToSettle(page);
     let viewport = await readTimelineViewport(page);
+    // Tests that hold the response need to observe the requested page before releasing it.
+    if (await loadingSpinner.isVisible()) {
+      return;
+    }
     if (viewport.scrollTop <= HISTORY_START_THRESHOLD_PX) {
       return;
     }
-    await expect(page.getByTestId("load-older-history-spinner")).toBeHidden();
+    await expect(loadingSpinner).toBeHidden();
     await waitForTimelineGeometryToSettle(page);
     viewport = await readTimelineViewport(page);
     expect(