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).

components/icons.tsx

@@ -237,3 +237,41 @@ export function RefreshIcon({ className }: IconProps) {
     </Svg>
   );
 }
+
+export function EyeIcon({ className }: IconProps) {
+  return (
+    <Svg className={className}>
+      <path d="M2.5 12S6 5.5 12 5.5 21.5 12 21.5 12 18 18.5 12 18.5 2.5 12 2.5 12Z" />
+      <circle cx="12" cy="12" r="3" />
+    </Svg>
+  );
+}
+
+export function EyeOffIcon({ className }: IconProps) {
+  return (
+    <Svg className={className}>
+      <path d="m3 3 18 18" />
+      <path d="M10.6 5.7c.5-.1.9-.2 1.4-.2 6 0 9.5 6.5 9.5 6.5a17.5 17.5 0 0 1-2.6 3.5" />
+      <path d="M6.6 6.8A16.6 16.6 0 0 0 2.5 12S6 18.5 12 18.5c1.4 0 2.7-.4 3.8-1" />
+      <path d="M9.9 9.9a3 3 0 0 0 4.2 4.2" />
+    </Svg>
+  );
+}
+
+export function QuestionIcon({ className }: IconProps) {
+  return (
+    <Svg className={className}>
+      <circle cx="12" cy="12" r="9" />
+      <path d="M9.7 9.4a2.5 2.5 0 0 1 4.9.7c0 1.7-2.5 2-2.5 3.5" />
+      <path d="M12 17h.01" />
+    </Svg>
+  );
+}
+
+export function XIcon({ className }: IconProps) {
+  return (
+    <Svg className={className}>
+      <path d="m6 6 12 12M18 6 6 18" />
+    </Svg>
+  );
+}