Fix initialization on headerless Steam CEF pages

[Acetab]

Problem

Steam's built-in browser can render store app pages without the legacy #global_header element. On these pages the content script may run before the client page has finished rendering, so the legacy page check fails and Augmented Steam does not initialize. Header-only widgets also assume that #global_action_menu and the legacy header are present.

Changes

• wait for the page check to become true instead of returning immediately when the content script starts before the page shell is available
• treat Steam client's .responsive_page_frame.no_header as a valid legacy page container
• defer store app initialization until app details or an error box is present
• safely skip the menu and warnings when the page has no compatible legacy-header mount point

The existing behavior on regular Steam web pages is unchanged: when the legacy header is available, the normal menu and warnings are mounted as before.

Testing

• npx tsc --noEmit
• npm run build chrome -- --production
• manually tested on a Steam client app page using Chromium 126

Related to #2307