fix: iframe/page preview height chain and toolbar UX

Problem: changing .preview-modal from height to max-height broke iframe
previews - <iframe style='flex:1'> needs a concrete ancestor height, which
max-height alone doesn't supply when content is shorter than the cap.

Fix - CSS:
  .preview-modal--full { height: 88vh } applied to non-tweet modals
  .preview-modal--full .preview-modal-body { max-height: none }
  .preview-iframe-toolbar span: remove text-transform/letter-spacing
    (was uppercasing the URL/title in shouty caps)

Fix - PreviewModal: className adds --full when entity_kind is not
  tweet/tweet_thread; tweet previews keep shrink-to-fit behavior.

Fix - PreviewPanel: pass title + original_url from summary to IframePreview
  for both HTML and PDF; wrappers use flex:1/minHeight:0 not height:100%.

Fix - IframePreview:
  - Accept title + originalUrl props; show originalUrl in toolbar (falls
    back to artifact src only when original_url absent); show title above
    URL when available
  - flex:1 + minHeight:0 instead of height:100% on the wrap div
  - Single unified layout for page + pdf (both just show the iframe)

frontend/src/components/PreviewModal.jsx

@@ -11,7 +11,15 @@ export default function PreviewModal({ archiveId, entry, detail, onClose }) {
 
   return (
     <div className="preview-modal-backdrop" onClick={onClose}>
-      <div className="preview-modal" onClick={e => e.stopPropagation()}>
+      <div
+        className={`preview-modal${
+          detail?.summary?.entity_kind === 'tweet' ||
+          detail?.summary?.entity_kind === 'tweet_thread'
+            ? ''
+            : ' preview-modal--full'
+        }`}
+        onClick={e => e.stopPropagation()}
+      >
         <div className="preview-modal-header">
           <span className="preview-modal-title">{entry?.title || entry?.entry_uid || 'Preview'}</span>
           <a