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/main.jsx

@@ -0,0 +1,8 @@
+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+import './styles.css'
+import App from './App'
+
+createRoot(document.getElementById('root')).render(
+  <StrictMode><App /></StrictMode>
+)