feat(ui): rewrite frontend in React with Vite

- Scaffold Vite+React project in frontend/ (bun, react 18, @vitejs/plugin-react)
- vite.config.js outputs to crates/archivr-server/static/ directly
- src/utils.js: formatBytes, valueText, formatTimestamp, SOURCE_ICONS, sourceIconSvg
- src/api.js: typed fetch wrappers for all API endpoints
- App.jsx: full state management, archive switching, debounced search,
  tag filter, view routing, capture dialog orchestration
- components/Topbar.jsx: archive switcher, nav, capture button
- components/CaptureDialog.jsx: native <dialog> ref with showModal/close,
  Escape key support, locator validation
- components/EntriesView.jsx + EntryRow.jsx: flex table with source icons,
  type pills, keyboard selection
- components/ContextRail.jsx: parallel detail+tags fetch, stale-race guard
  via selectSeqRef, inline tag assign/remove
- components/RunsView.jsx: runs kept as <table> (matches original)
- components/AdminView.jsx: mounted archives list
- components/TagsView.jsx: recursive tag tree with active state
- Remove old app.js; styles.css moved to frontend/src/ (Vite bundles it)

frontend/src/components/AdminView.jsx

@@ -0,0 +1,15 @@
+export default function AdminView({ archives }) {
+  return (
+    <section id="admin-view" className="view admin-view is-active">
+      <h1>Mounted Archives</h1>
+      <div className="admin-list">
+        {archives.map(archive => (
+          <div key={archive.id} className="admin-archive">
+            <strong>{archive.label}</strong>
+            <div className="muted">{archive.archive_path}</div>
+          </div>
+        ))}
+      </div>
+    </section>
+  )
+}