Move the panel background stacking context off partial-panel-resolver

Since #54801 the resolver creates a stacking context (isolation: isolate) so
its fixed ::before background sits behind the panel. The resolver is rendered
conditionally, once the startup data is available, and isolating it can
intermittently leave the Android WebView accessibility tree (Chromium
109-124) with an empty panel subtree while the page paints normally. UI
automation then never sees the dashboard; the companion app nightly E2E has
hit this on the legacy build (API 32/33/35) since the change landed.

Move the isolation to home-assistant-main's host, which exists before the
resolver is rendered. The pseudo-element and the Lovelace view background
both stay at z-index -1 in that context, in the same order as before, so the
themed background behaves exactly as #54801 intended. Measured with Maestro
on a Chrome 109 emulator: 0 stale trees in 62 cold loads, against 11 in 100
with the isolation on the resolver.

src/layouts/home-assistant-main.ts

@@ -180,6 +180,13 @@ export class HomeAssistantMain extends LitElement {
       color: var(--primary-text-color);
       /* remove the grey tap highlights in iOS on the fullscreen touch targets */
       -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
+      /* Keep the negative z-index panel backgrounds (the resolver's ::before
+         and the Lovelace view background) in this host's stacking context.
+         Isolating the conditionally rendered partial-panel-resolver instead
+         can intermittently leave the Android WebView (Chromium 109-124)
+         accessibility tree with an empty panel subtree while the page paints
+         normally, which breaks UI automation such as the companion app E2E. */
+      isolation: isolate;
       --ha-sidebar-width: calc(56px + var(--safe-area-inset-left, 0px));
       --ha-top-app-bar-width: calc(100% - var(--ha-sidebar-width));
       --safe-area-content-inset-left: 0px;
@@ -201,7 +208,6 @@ export class HomeAssistantMain extends LitElement {
     partial-panel-resolver {
       display: block;
       height: 100%;
-      isolation: isolate;
     }
     partial-panel-resolver::before {
       content: "";