Initial commit — AI-powered coding tutor (Professor)

Next.js 16, React 19, Monaco editor, Anthropic SDK, multi-provider AI,
Wandbox Python execution, iframe HTML preview, SQLite auth + session persistence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/pistonClient.ts

@@ -0,0 +1,57 @@
+import type { ExecutionResult } from '@/types';
+
+// Wandbox — free public code execution API, no auth required
+// https://github.com/melpon/wandbox
+const WANDBOX_API = 'https://wandbox.org/api/compile.json';
+const TIMEOUT_MS = 15_000;
+
+export async function executePython(code: string, stdin = ''): Promise<ExecutionResult> {
+  const controller = new AbortController();
+  const timer = setTimeout(() => controller.abort(), TIMEOUT_MS);
+
+  try {
+    const res = await fetch(WANDBOX_API, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      signal: controller.signal,
+      body: JSON.stringify({
+        compiler: 'cpython-3.12.7',
+        code,
+        stdin,
+      }),
+    });
+
+    if (!res.ok) {
+      throw new Error(`Execution service error: ${res.status}`);
+    }
+
+    const data = await res.json();
+    const exitCode = parseInt(data.status ?? '0', 10);
+
+    // Wandbox separates compiler errors from runtime errors
+    const stderr = [data.program_error, data.compiler_error]
+      .filter(Boolean)
+      .join('\n')
+      .trim();
+
+    return {
+      stdout: data.program_output ?? '',
+      stderr,
+      exitCode,
+      timedOut: !!data.signal,
+    };
+  } catch (err: unknown) {
+    if (err instanceof Error && err.name === 'AbortError') {
+      return { stdout: '', stderr: '', exitCode: -1, timedOut: true, error: 'Execution timed out after 15 seconds.' };
+    }
+    return {
+      stdout: '',
+      stderr: '',
+      exitCode: -1,
+      timedOut: false,
+      error: err instanceof Error ? err.message : 'Unknown execution error',
+    };
+  } finally {
+    clearTimeout(timer);
+  }
+}