fix(dashboard): inject improvement modal via head config, not _footer.md

_footer.md is not a supported special file in Observable Framework 1.13.3
and was silently ignored. The preview server does serve src/*.js files at
their root-relative path, so the correct approach is a <script type="module">
in the head config — runs once on page load, persists across SPA navigation.
Removed _footer.md.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/src/_footer.md

@@ -1,6 +0,0 @@
-```js
-// Right-click improvement modal — initialised once, active on every page
-import {initImprovementModal} from "./components/improvement-modal.js";
-import {API} from "./components/config.js";
-initImprovementModal({apiBase: API, domain: "custodian"});
-```