Add the Next.js web app and shared lesson content.

Move lessons out of the C sources into JSON so the web client can use the same material as the desktop app, and point release builds at desktop/.

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

web/src/components/exercises/TaskShell.tsx

@@ -0,0 +1,69 @@
+'use client';
+
+import { useApp } from '@/app/providers';
+
+export type CheckState = 'idle' | 'ok' | 'wrong';
+
+/** The instruction lines every exercise takes from its task. */
+export interface TaskFrame {
+  subtitle?: string;
+  /** Worked example shown above the items. */
+  example?: string;
+  /** Extra guidance under the instruction, e.g. a word bank. */
+  note?: string;
+}
+
+/**
+ * Frame shared by every exercise: the instruction, the task body, a check
+ * button and the pass/fail feedback line.
+ */
+export function TaskShell({
+  subtitle,
+  example,
+  note,
+  state,
+  checkLabel,
+  onCheck,
+  onReveal,
+  revealed,
+  children,
+}: TaskFrame & {
+  state: CheckState;
+  /** i18n key overriding the default "check" button label. */
+  checkLabel?: string;
+  onCheck: () => void;
+  /** Provided by exercises that can show a model solution. */
+  onReveal?: () => void;
+  revealed?: boolean;
+  children: React.ReactNode;
+}) {
+  const { t } = useApp();
+
+  return (
+    <div className="stack">
+      {subtitle && <p className="page__sub">{subtitle}</p>}
+      {note && <p className="ex__note">{note}</p>}
+      {example && <p className="ex__example">{example}</p>}
+
+      <div className="ex">{children}</div>
+
+      <div className="actions">
+        <button className="btn" onClick={onCheck}>
+          {t(checkLabel ?? 'check')}
+        </button>
+
+        {onReveal && (
+          <button className="btn btn--ghost" onClick={onReveal} disabled={revealed}>
+            {t('net_solution')}
+          </button>
+        )}
+      </div>
+
+      {state !== 'idle' && (
+        <p className={`feedback feedback--${state === 'ok' ? 'ok' : 'err'}`} role="status">
+          {t(state === 'ok' ? 'feedback_ok' : 'feedback_retry_short')}
+        </p>
+      )}
+    </div>
+  );
+}