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/Topbar.jsx

@@ -0,0 +1,20 @@
+export default function Topbar({ archives, archiveId, onArchiveChange, view, onViewChange, onCaptureClick }) {
+  return (
+    <header className="topbar">
+      <div className="brand">Archivr</div>
+      <select className="archive-switcher" aria-label="Select archive"
+        value={archiveId ?? ''} onChange={e => onArchiveChange(e.target.value)}>
+        {archives.map(a => <option key={a.id} value={a.id}>{a.label}</option>)}
+      </select>
+      <nav className="nav" aria-label="Primary">
+        {['archive', 'runs', 'admin', 'tags'].map(name => (
+          <button key={name} className={`nav-link${view === name ? ' is-active' : ''}`}
+            onClick={() => onViewChange(name)}>
+            {name.charAt(0).toUpperCase() + name.slice(1)}
+          </button>
+        ))}
+      </nav>
+      <button className="capture-button" onClick={onCaptureClick}>+ Capture</button>
+    </header>
+  )
+}