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/observablehq.config.js

@@ -75,6 +75,10 @@ export default {
   ],
   theme: ["air", "near-midnight"],
   head: `<link rel="icon" href="data:image/svg+xml,<svg xmlns=%22http://www.w3.org/2000/svg%22 viewBox=%220 0 100 100%22><text y=%22.9em%22 font-size=%2290%22>🗄️</text></svg>">
+<script type="module">
+import {initImprovementModal} from "/components/improvement-modal.js";
+initImprovementModal({apiBase: "http://127.0.0.1:8000", domain: "custodian"});
+</script>
 <style>
 .kpi-infobox { background: var(--theme-background-alt, #f9f9f9); border: 1px solid var(--theme-foreground-faint, #e0e0e0); border-radius: 10px; padding: 0.75rem 1rem; position: relative; box-shadow: 0 1px 6px rgba(0,0,0,0.07); margin-bottom: 1.25rem; }
 .kpi-infobox-title { font-size: 0.68rem; font-weight: 700; text-transform: uppercase; letter-spacing: 0.08em; color: var(--theme-foreground-muted, #888); margin-bottom: 0.55rem; padding-right: 1.6rem; }

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"});
-```