feat(posts): plain bold titles; reserve View-original to the post card; provenance links to the posts feed

- Post titles arrived as stored HTML (e.g. <strong>…</strong>) rendering as
  literal markup. New toPlainText() strips tags; titles render plain + bold
  (ProvenancePanel and PostCard).
- Removed "View original post" from the provenance panel (modal) — the
  open-original button lives on the PostCard (the post view).
- Provenance "View post" now navigates to the /posts feed (post_id query),
  not the gallery image grid. (Feed in-context landing lands next.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/utils/htmlSanitize.js

@@ -29,6 +29,17 @@ export function sanitizeHtml (html) {
   return doc.body.innerHTML
 }
 
+// Strip all tags + decode entities to plain text. Used for titles that
+// arrive as stored HTML (e.g. "<strong>Edelgard at the Sex-Arcade</strong>")
+// which must render as text, not literal markup. DOMParser yields an inert
+// document (no script execution, no resource loads), so reading textContent
+// is safe.
+export function toPlainText (html) {
+  if (typeof html !== 'string' || !html) return ''
+  const doc = new DOMParser().parseFromString(html, 'text/html')
+  return (doc.body.textContent || '').trim()
+}
+
 function _scrubNode (node) {
   const children = Array.from(node.children)
   for (const child of children) {