Let the app run under a panel page's document base

A panel page loads the app with <base> pointing at the app's server, so
addresses the app builds resolve against the document base instead of
the page location, and hash updates stay on the page's own address.
The dev server answers panel origins with CORS on the API routes too.

src/tooling/viteBuild.test.mjs

@@ -12,7 +12,8 @@ test('explicit build inputs preserve browser-only defines, plugin order and loop
     googleApiKey: 'browser-fixture',
     cesiumToken: 'ion-fixture',
   });
-  assert.equal(config.plugins[2], plugin);
+  assert.equal(config.plugins[0].name, 'panel-cors');
+  assert.equal(config.plugins[3], plugin);
   assert.equal(config.server.host, 'localhost');
   assert.equal(config.server.port, 4173);
   assert.deepEqual(config.server.allowedHosts, [
@@ -42,6 +43,40 @@ test('explicit build inputs preserve browser-only defines, plugin order and loop
   );
 });
 
+test('panel origins may read every dev server path, other sites may not', () => {
+  let middleware;
+  createBrowserViteConfig().plugins[0].configureServer({
+    middlewares: { use: (fn) => (middleware = fn) },
+  });
+  const request = (origin, method = 'GET') => {
+    const headers = {};
+    let ended = false;
+    let passed = false;
+    middleware(
+      { method, headers: origin ? { origin } : {} },
+      {
+        setHeader: (name, value) => (headers[name] = value),
+        end: () => (ended = true),
+      },
+      () => (passed = true),
+    );
+    return { headers, ended, passed };
+  };
+  const panel = request('https://abc123.claudemcpcontent.com');
+  assert.equal(
+    panel.headers['Access-Control-Allow-Origin'],
+    'https://abc123.claudemcpcontent.com',
+  );
+  assert.equal(panel.passed, true);
+  assert.equal(
+    request('https://abc123.claudemcpcontent.com', 'OPTIONS').ended,
+    true,
+  );
+  const other = request('https://example.com');
+  assert.deepEqual(other.headers, {});
+  assert.equal(other.passed, true);
+});
+
 test('build helper does not discover environment values or construct local providers', () => {
   const before = process.env.GOOGLE_MAPS_API_KEY;
   process.env.GOOGLE_MAPS_API_KEY = 'environment-fixture';
@@ -52,7 +87,7 @@ test('build helper does not discover environment values or construct local provi
       undefined,
     );
     assert.deepEqual(
-      config.plugins.slice(2).map((plugin) => plugin.name),
+      config.plugins.slice(3).map((plugin) => plugin.name),
       ['embed-framing'],
     );
   } finally {
@@ -66,7 +101,7 @@ test('root config retains existing named exports and standalone provider order',
     assert.equal(compatibility[name], value, name);
   const config = standaloneConfig({ mode: 'test' });
   assert.deepEqual(
-    config.plugins.slice(2, -3).map((plugin) => plugin.name),
+    config.plugins.slice(3, -3).map((plugin) => plugin.name),
     providers.localProviderPlugins().map((plugin) => plugin.name),
   );
   assert.equal(config.plugins.at(-4).name, 'gev-key-setup');