perf(frontend): split chat streaming, load Inter, isolate admin CSS

Settled messages no longer rebuild on every token, Inter is actually
requested, and admin routes drop the 33 KB chat stylesheet. Root
force-dynamic is gone so public shells can prerender without changing
the no-store Cache-Control contract.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/admin/admin.css

@@ -1,3 +1,12 @@
+:root {
+  --font-body: StyreneB, var(--font-inter, Inter), system-ui, sans-serif;
+  --font-mono: "JetBrains Mono", "SFMono-Regular", Consolas, monospace;
+}
+html, body {
+  width: 100%;
+  min-height: 100%;
+  margin: 0;
+}
 .admin-app-shell {
   min-height: 100dvh;
   background: #f3f2ee;