feat: Sprint 1 — Playwright e2e + sandbox orchestration (TDD)

- e2e: Playwright chat-flow (sign up -> chat) with api/web webServer harness
- api: sandbox services subset/media-proxy/manifest/gc + DockerClient contract
- api: SandboxManager orchestrator (fake-Docker tested); /health route
- 53 unit tests green; Playwright e2e green

api/__tests__/sandbox/manifest.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { diffManifest } from '../../src/sandbox/manifest.ts';
+
+describe('diffManifest', () => {
+  it('detects added, changed, and removed paths', () => {
+    const before = {
+      '/theme/style.css': 'a1',
+      '/theme/theme.json': 'b2',
+      '/theme/old.css': 'c3',
+    };
+    const after = {
+      '/theme/style.css': 'a1',
+      '/theme/theme.json': 'b2-new',
+      '/theme/new.css': 'd4',
+    };
+
+    expect(diffManifest(before, after)).toEqual({
+      added: ['/theme/new.css'],
+      changed: ['/theme/theme.json'],
+      removed: ['/theme/old.css'],
+    });
+  });
+
+  it('reports empty arrays when nothing changed', () => {
+    const manifest = { '/a.css': 'x' };
+    expect(diffManifest(manifest, { ...manifest })).toEqual({ added: [], changed: [], removed: [] });
+  });
+});