Viewer initialization fails on forks after footer simplification

## Problem

The data viewer stops during initialization on staging and fork GitHub Pages URLs. `applyRepoContext()` writes to `footer-nonproduction-note`, but that element was removed by #164. The resulting null dereference prevents CSV discovery and table initialization.

## Reproduction

Load current `index.html` with a non-production repository URL, for example `https://example.github.io/dotgov-data/`. Initialization raises:

```text
TypeError: Cannot set properties of null (setting 'hidden')
```

Reproduced from `04d6a886c3c6c33905bfd67e68a1b5ab45779f4e`. Production URLs bypass the failing branch. The same error also prevents the missing-CDN-library message from appearing on forks.

## Expected behavior

Remove the stale footer-element access while retaining the non-production banner, title and production-note handling. Forks should proceed to CSV discovery, or show the existing library-load error when dependencies are unavailable.

Start in index.html at applyRepoContext() and inspect the non-production URL branch that accesses footer-nonproduction-note. Load the page with the example fork URL and verify initialization reaches CSV discovery, while the non-production banner, title, production-note handling, and existing missing-library error remain functional.

Written by the indexing model from the issue text.