feat(frontend): restyle ThothII to the Omics Portal (GSD) design system

Adopt the portal's design language so the app embeds seamlessly into its
header/sidebar chrome later:
- Token layer rewritten as raw OKLCH triplets (the previous oklch()-wrapped
  vars produced invalid colors; the UI was effectively unstyled). Values mirror
  GSD: Manrope + Fraunces fonts, #cb333b red brand, warm off-white surfaces,
  pill buttons, brand focus rings. Dark tokens follow .dark or the portal's
  [data-bs-theme=dark].
- Layout: no left sidebar, no header (the portal supplies them). Conversation
  column flush-left with a slim F1-F8 phase strip; session rail moved to the
  RIGHT, mirroring the portal's left sidebar (red section label, red-tint active
  item). Editorial empty state.
- Transcript component now actually renders the streamed assistant turns (the
  store accumulated them but nothing displayed them). Toaster mounted for errors
  (matchMedia polyfill added to the jsdom test setup).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/SteerInput.tsx

@@ -1,6 +1,8 @@
 // frontend/src/shell/SteerInput.tsx
 import { useState } from "react";
+import { ArrowUpIcon } from "lucide-react";
 import { postSteer } from "../api/sessions";
+import { Button } from "../components/ui/button";
 
 interface Props {
   sessionId: string;
@@ -24,21 +26,24 @@ export function SteerInput({ sessionId }: Props) {
   }
 
   return (
-    <div className="flex gap-2 items-center border-t pt-2">
+    <div className="flex items-center gap-2 rounded-full border border-input bg-card px-2 py-1.5 shadow-sm transition-colors focus-within:border-primary/40 focus-within:ring-3 focus-within:ring-ring/15">
       <input
         type="text"
         value={text}
         onChange={(e) => setText(e.target.value)}
         onKeyDown={handleKeyDown}
         placeholder="Steering libero (!)…"
-        className="flex-1 border rounded px-2 py-1 text-sm focus:outline-none focus:ring-1"
+        aria-label="Steering"
+        className="flex-1 bg-transparent px-2 text-sm outline-none placeholder:text-muted-foreground"
       />
-      <button
+      <Button
+        size="icon-sm"
         onClick={submit}
-        className="border rounded px-3 py-1 text-sm hover:bg-accent"
+        disabled={!text.trim()}
+        aria-label="Invia"
       >
-        Invia
-      </button>
+        <ArrowUpIcon />
+      </Button>
     </div>
   );
 }