feat: entry previews — tweet/thread/article/video/audio/image/iframe/pdf

- PreviewModal: modal overlay with new-tab link (↗) and keyboard close
- PreviewPanel: routes by entity_kind + primary_media extension to the
  correct viewer (tweet/video/audio/pdf/html/image/fallback)
- TweetPreview: full X-style tweet, thread, and article renderer with
  local artifact map for archived media (CDN fallback)
- AudioBar: persistent fixed bottom player, triggered via ContextRail
  Play button; body.has-audio-bar pads content above it
- VideoPreview, IframePreview, ImagePreview: inline viewers
- PreviewPage: standalone /preview/:archiveId/:entryUid route
- ContextRail: Play/Preview buttons; isAudio/isPreviewable detection
- App.jsx: preview modal state, currentAudio state, preview route guard,
  has-audio-bar body class effect
- routes.rs: CSP updated (media-src self blob https; frame-ancestors self;
  Google Fonts + external images/scripts whitelisted)
- styles.css: preview modal, tweet-wrap scroll (min-height:0), audio bar
  body padding, newtab button, preview panel flex layout

frontend/src/components/PreviewModal.jsx

@@ -0,0 +1,32 @@
+import { useEffect } from 'react'
+import PreviewPanel from './PreviewPanel'
+
+export default function PreviewModal({ archiveId, entry, detail, onClose }) {
+  // Close on Escape key
+  useEffect(() => {
+    const handler = (e) => { if (e.key === 'Escape') onClose() }
+    window.addEventListener('keydown', handler)
+    return () => window.removeEventListener('keydown', handler)
+  }, [onClose])
+
+  return (
+    <div className="preview-modal-backdrop" onClick={onClose}>
+      <div className="preview-modal" onClick={e => e.stopPropagation()}>
+        <div className="preview-modal-header">
+          <span className="preview-modal-title">{entry?.title || entry?.entry_uid || 'Preview'}</span>
+          <a
+            className="preview-modal-newtab"
+            href={`/preview/${archiveId}/${entry?.entry_uid}`}
+            target="_blank"
+            rel="noopener noreferrer"
+            title="Open in new tab"
+          >↗</a>
+          <button className="preview-modal-close" onClick={onClose} aria-label="Close preview">×</button>
+        </div>
+        <div className="preview-modal-body">
+          <PreviewPanel archiveId={archiveId} entry={entry} detail={detail} />
+        </div>
+      </div>
+    </div>
+  )
+}