fix(e2e): select the measured wrapper by data-slot, not by birth order

The navigation-geometry spec failed on a merge commit whose tree was
byte-identical to the branch head that had passed minutes earlier, so nothing had
regressed. The spec resolved the element it measures with
`main.firstElementChild`, independently in each of its two `page.evaluate` calls.
Any node mounting ahead of the wrapper between the two reads made it measure two
different elements, which is why the failure was 64 against 131.92 rather than a
two-pixel drift. A timing problem gives a small difference or a null; this was a
substitution.

The shell's content wrapper now carries a stable `data-slot`, both reads select
it by that, and the wait gates on the element rather than on scroll state alone.
The same positional pattern in the overscroll spec is fixed with it, so the class
is gone from the measurement paths rather than from one spec.

Re-running the job made it green, which is a workaround. This is the fix.

src/components/layout/auth-shell.tsx

@@ -335,7 +335,10 @@ export function AuthShell({
               the desktop bottom-6 anchor), so the last line of content
               can always scroll out from under the floating button.
             */}
-            <div className="mx-auto max-w-screen-xl px-4 pt-6 pb-20 md:px-6">
+            <div
+              data-slot="main-content-wrapper"
+              className="mx-auto max-w-screen-xl px-4 pt-6 pb-20 md:px-6"
+            >
               {children}
             </div>
           </main>