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/LibraryScreen.tsx

@@ -3,7 +3,6 @@
 import Link from "next/link";
 import { useMemo, useRef, useState, useSyncExternalStore } from "react";
 
-import { ApiKeyBanner } from "@/components/ApiKeyBanner";
 import { DeckCard } from "@/components/DeckCard";
 import { FolderBar } from "@/components/FolderBar";
 import { FolderIcon, UploadIcon } from "@/components/icons";
@@ -168,7 +167,6 @@ export function LibraryScreen() {
           }}
         />
 
-        <ApiKeyBanner />
 
         {message ? (
           <p