test: wait until the page shows a screen, not only until it lays out (closes #502)

The browser suites' wait for a screen passed as soon as the element laid
out. Until the page's stylesheet has applied, every view lays out, so an
approval test could read the prompt before the page's script had filled
it. visible() in the Chrome harness and waitVisible() in the Firefox
driver now also wait for the page to finish loading and for neither the
element nor anything around it to carry the hidden class that showView()
keeps on every view but the current one. No caller changed.

Model: opus-5-5

TODO.md

@@ -44,6 +44,16 @@ then continue tagging as milestones land.
 
 # Completed Steps
 
+- 2026-10-08: The browser suites no longer read a screen before the page has
+  shown it ([#502](https://git.eeqj.de/sneak/AutistMask/issues/502)). Their wait
+  for a screen used to pass as soon as the element laid out, which every view
+  does until the page's stylesheet has applied, so an approval test could read
+  the prompt's fields before the page's script had filled them. `visible()` in
+  `tests/e2e/harness.js` and `waitVisible()` in `tests/e2e/firefox/driver.js`
+  now also wait for the page to finish loading and for neither the element nor
+  anything around it to carry the `hidden` class that `showView()` puts on every
+  view but the current one. No caller changed.
+
 - 2026-10-07: A site can no longer switch the wallet's network by itself
   ([#408](https://git.eeqj.de/sneak/AutistMask/issues/408)). A connected site's
   `wallet_switchEthereumChain` request for the other supported network opens a