v0.30.2: header nav label "Philosophy" -> "About"

Revert the §14.3 persistent chrome link's display text from "Philosophy"
back to "About" (relabeled in v0.21.0). Display text only — route
/philosophy, the header-about class, and the page are unchanged. Patch
per SPEC §20.2: cosmetic, no deployment action beyond a frontend rebuild.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/App.jsx

@@ -213,7 +213,7 @@ export default function App() {
               wonders why a conversation is public can reach the answer
               in two clicks. Anonymous viewers see it too. */}
           <Link to="/philosophy" className="header-about" title="Why this exists (§14)">
-            Philosophy
+            About
           </Link>
           <Link to="/docs" className="header-about" title="User guide">
             Docs