Rename landing page to Open Human Model with Wiggleverse RFC attribution

The 5dbcac8 reframe established OHM as the corpus and Wiggleverse RFC
as the substrate. The landing page and the global header still read
"Wiggleverse RFCs," which puts the substrate's name in front of a
visitor when what they're actually visiting is OHM. Flip both surfaces.

- App header brand and Landing's H1 → "Open Human Model"
- Subtitle becomes "An open dictionary of the words humans and
  machines need to agree on" — names the corpus, not the process tool
- Pitch paragraph reworded to lead with "the Open Human Model is the
  corpus this framework produces," with first RFC defining "human"
- New .landing-attribution block at the foot of the deck: "OHM is run
  by Ben on Wiggleverse RFC — the open-source standardization-process
  software that powers it. Anyone can run their own collaboration on
  the same substrate." Reader can now tell instance-vs-substrate at
  a glance

frontend/src/App.jsx

@@ -84,7 +84,7 @@ export default function App() {
     <div className="app">
       <header className="app-header">
         <div className="app-brand">
-          <Link to="/">Wiggleverse RFCs</Link>
+          <Link to="/">Open Human Model</Link>
         </div>
         <div className="header-right">
           {/* §14.3: the persistent About link. One word, no badge, no