refactor(frontend): extract full-page loading screen controller (@hunterpaulson)

[hunterpaulson]

Description

• extract the full-page loading screen orchestration from page-controller into a dedicated loading-screen controller
• update page-controller to use the extracted helper

Why

The full-page loading screen logic was previously embedded inside page-controller, which made it harder to reuse outside route transitions.
This refactor keeps the current behavior the same while moving the loading-screen orchestration into a dedicated controller so future non-routing flows (e.g. funboxes) can reuse the same loading UI without duplicating controller internals.

Validation

• pnpm ts-check
• pnpm lint-fast "src/ts/controllers/loading-screen.ts" "src/ts/controllers/page-controller.ts"
• manually verified the extracted helper still shows the full-page loading screen with both bar and spinner modes

[fehmer]

hi @hunterpaulson, thank you for your contribution.

Do you already have a use-case in mind?

[hunterpaulson]

@fehmer yes, I’m working on a funbox that needs to do a second or two of initialization the first time it is selected, and I wanted to reuse the existing full-page progress bar for that flow instead of the LoaderBar that doesn't currently support progress.