fix: Switch preview gateway from path-prefix to subdomain routing

Path-prefix routing (`localhost:9100/<id>/`) broke SPAs because absolute
asset paths (`/assets/index.js`) didn't match the `handle_path /<id>/*`
route. Subdomain routing (`<id>.localhost:9100/`) resolves this since
all paths are relative to the root. Chrome/Firefox resolve *.localhost
to 127.0.0.1 natively — no DNS setup needed.

apps/server/preview/manager.test.ts

@@ -220,7 +220,7 @@ describe('PreviewManager', () => {
       expect(result.projectId).toBe('proj-1');
       expect(result.branch).toBe('feature-x');
       expect(result.gatewayPort).toBe(9100);
-      expect(result.url).toBe('http://localhost:9100/abc123test/');
+      expect(result.url).toBe('http://abc123test.localhost:9100/');
       expect(result.mode).toBe('preview');
       expect(result.status).toBe('running');
 
@@ -233,7 +233,7 @@ describe('PreviewManager', () => {
       expect(buildingEvent).toBeDefined();
       expect(readyEvent).toBeDefined();
       expect((readyEvent!.payload as Record<string, unknown>).url).toBe(
-        'http://localhost:9100/abc123test/',
+        'http://abc123test.localhost:9100/',
       );
     });
 
@@ -472,7 +472,7 @@ describe('PreviewManager', () => {
       expect(previews).toHaveLength(2);
       expect(previews[0].id).toBe('aaa');
       expect(previews[0].gatewayPort).toBe(9100);
-      expect(previews[0].url).toBe('http://localhost:9100/aaa/');
+      expect(previews[0].url).toBe('http://aaa.localhost:9100/');
       expect(previews[0].mode).toBe('preview');
       expect(previews[0].services).toHaveLength(1);
       expect(previews[1].id).toBe('bbb');
@@ -573,7 +573,7 @@ describe('PreviewManager', () => {
       expect(status!.status).toBe('running');
       expect(status!.id).toBe('abc');
       expect(status!.gatewayPort).toBe(9100);
-      expect(status!.url).toBe('http://localhost:9100/abc/');
+      expect(status!.url).toBe('http://abc.localhost:9100/');
       expect(status!.mode).toBe('preview');
     });