Show loading spinner throughout the full app boot sequence

[nbudin]

Purpose

There were two gaps in loading feedback during app boot: a blank screen before the JS bundle downloaded, and a ~1 second blank screen between the bootstrap Suspense spinner disappearing and the route components appearing on screen.

The root cause of the second gap was subtle: createBrowserRouter starts the initial navigation immediately when called, but router.state.initialized is false until the navigation completes. During that window, RouterProvider renders nothing visible because the lazy root module and its loaders haven't finished yet — and the existing Suspense spinner had already gone away.

Changes

💻 Engineer-facing

• Inline HTML spinner in the SPA shell and CMS-rendered AppRoot so there's something on screen as soon as the HTML parses, before any JS runs. Extracted into ApplicationHelper#app_root_loading_content so both cdn_spa_shell.html.erb and CmsRenderingContext use the same markup.
• RouterLoadingOverlay component rendered alongside RouterProvider. It subscribes to router.state.initialized via useLayoutEffect (runs before RouterProvider's own layout effect, so it sees the buffered post-navigation state) and removes itself once the router is ready. No full-page overlay — just the same Bootstrap spinner, in normal flow, so it disappears at the same time content appears.

Testing

Tested locally across cold loads and navigations. The spinner now appears immediately and stays visible until the page is ready.

🚢

🤖 Generated with Claude Code

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
app/helpers/application_helper.rb	🟢 82.61%	🟢 85.19%	🟢 2.58%
app/javascript/BuiltInFormControls/AddFileModal.tsx	🔴 33.33%	🔴 0%	🔴 -33.33%
app/javascript/BuiltInFormControls/LiquidInput.tsx	🟠 60%	🟠 57.5%	🔴 -2.5%
app/javascript/setupI18Next.ts	🟠 72.41%	🟠 65.52%	🔴 -6.89%
Overall Coverage	🟢 52.82%	🟢 52.78%	🔴 -0.04%

Minimum allowed coverage is 0%, this run produced 52.78%