Add full-screen API-key gate

In user-key mode a browser with no stored Ollama key now gets a polished
gate instead of the app shell: one masked input with a show/hide toggle,
a single Continue button, inline validation errors, and a "?" help sheet
with the steps to a free key.

- Validates the draft key through /api/health (real 1-token probe) before
  saving; invalid-key and network failures read differently.
- Key still lives only in this browser's localStorage; nothing server-side.
- Gate wraps /, /create and deck study; server-key and local-daemon modes
  skip it; removing the key in Settings brings it back.
- Removes the now-redundant dashboard/create API-key banner.
- Monochrome corner glows (dark near-black / light near-white), 24px card,
  48px field and button, gentle fade/slide-in, reduced-motion aware.
- Help modal: centred dialog on desktop, bottom sheet on mobile, closes via
  X, Escape or backdrop.
- README and browser suites updated (34 gate checks, 15 key-flow checks,
  12 shipped-default checks; ui/folders/docs/pwa/security/contrast green).

app/create/page.tsx

@@ -1,3 +1,4 @@
+import { ApiKeyGate } from "@/components/ApiKeyGate";
 import { AppHeader } from "@/components/AppHeader";
 import { MobileSidebar, Sidebar, SidebarOffset } from "@/components/Sidebar";
 import { TabBar } from "@/components/TabBar";
@@ -9,17 +10,19 @@ export const metadata = {
 
 export default function CreatePage() {
   return (
-    <div className="flex min-h-dvh bg-[var(--page)]">
-      <Sidebar />
-      <SidebarOffset />
-      <MobileSidebar />
-      <main className="min-w-0 flex-1">
-        <AppHeader />
-        <div className="mx-auto w-full max-w-3xl px-4 pb-cta lg:px-10 lg:pb-20">
-          <UploadScreen />
-        </div>
-        <TabBar />
-      </main>
-    </div>
+    <ApiKeyGate>
+      <div className="flex min-h-dvh bg-[var(--page)]">
+        <Sidebar />
+        <SidebarOffset />
+        <MobileSidebar />
+        <main className="min-w-0 flex-1">
+          <AppHeader />
+          <div className="mx-auto w-full max-w-3xl px-4 pb-cta lg:px-10 lg:pb-20">
+            <UploadScreen />
+          </div>
+          <TabBar />
+        </main>
+      </div>
+    </ApiKeyGate>
   );
 }