perf: Chromium 启动 flag 微调 + screencast 画质 q60->q50

Flag：加 --no-first-run、--no-default-browser-check、
--disable-default-apps、--disable-extensions、
--disable-background-networking、--disable-sync、
--disable-translate，只跳过首跑/装配活，不改渲染与工具
行为。实测（runsc 容器，launch + new_page，各 3 次交替）：
旧中位数 0.889s、新 0.819s——只差 70ms 且在逐次噪声
（0.70-0.99s）里，几百毫秒的预期不成立；零成本顺手留着。
画质：screencast 与 eager 抓帧 q60 -> q50，分辨率
1280x800 不动，点击坐标系不受影响。实测同页文本+渐变：
62068 -> 56428 字节（-9%），不是经验法则的 30%；觉得糊
一句话改回 60。

cloud/playwright_browser.py

@@ -83,9 +83,23 @@ class Browser:
             else:
                 from playwright.sync_api import sync_playwright
                 pw = sync_playwright().start()
+            # Extra flags skip first-run/setup work only; they
+            # change nothing about page rendering or tool
+            # behaviour. Measured 2026-10-30 (runsc container,
+            # launch + new_page, 3 runs each, alternating):
+            # old median 0.889s, new median 0.819s -- ~70ms,
+            # inside run-to-run noise (0.70-0.99s either way).
+            # Kept anyway: zero cost, directionally faster.
             launch = {"args": ["--no-sandbox",
                                "--disable-dev-shm-usage",
-                               "--disable-gpu"]}
+                               "--disable-gpu",
+                               "--no-first-run",
+                               "--no-default-browser-check",
+                               "--disable-default-apps",
+                               "--disable-extensions",
+                               "--disable-background-networking",
+                               "--disable-sync",
+                               "--disable-translate"]}
             if self.proxy:
                 launch["proxy"] = {"server": self.proxy}
             browser = pw.chromium.launch(**launch)
@@ -318,8 +332,13 @@ class Browser:
             # bytes), so dropping the frame-skip buys
             # responsiveness exactly when it matters -- scrolling,
             # dragging, animation -- without a static-page tax.
+            # quality 50 (was 60): measured 2026-10-30 on a
+            # text+gradient page, 62068 -> 56428 bytes (-9%).
+            # Less than the 30% rule of thumb, but free: same
+            # 1280x800 pixels, so click coordinates are
+            # unaffected. Revert to 60 if text looks soft.
             cdp.send("Page.startScreencast",
-                     {"format": "jpeg", "quality": 60,
+                     {"format": "jpeg", "quality": 50,
                       "maxWidth": 1280, "maxHeight": 800,
                       "everyNthFrame": 1})
             self._cdp = cdp
@@ -343,7 +362,7 @@ class Browser:
         try:
             result = self._cdp.send(
                 "Page.captureScreenshot",
-                {"format": "jpeg", "quality": 60})
+                {"format": "jpeg", "quality": 50})
             data = result.get("data") if isinstance(result, dict) \
                 else None
             if data:

cloud/tests/test_playwright.py

@@ -180,6 +180,11 @@ class BrowserTest(unittest.TestCase):
                          {"server": "http://proxy:8888"})
         self.assertIn("--no-sandbox",
                       self.browser._pw.launch_kwargs["args"])
+        for flag in ("--no-first-run", "--no-default-browser-check",
+                     "--disable-default-apps", "--disable-extensions",
+                     "--disable-background-networking", "--disable-sync",
+                     "--disable-translate"):
+            self.assertIn(flag, self.browser._pw.launch_kwargs["args"])
 
     def test_snapshot_truncates_with_flag(self):
         self.browser.navigate("https://example.com")
@@ -254,7 +259,7 @@ class BrowserTest(unittest.TestCase):
         self.browser.set_frame_sink(sink)
         self.browser.navigate("https://example.com")
         self.assertIn(("Page.startScreencast",
-                       {"format": "jpeg", "quality": 60,
+                       {"format": "jpeg", "quality": 50,
                         "maxWidth": 1280, "maxHeight": 800,
                         "everyNthFrame": 1}),
                       [(m, p) for m, p in self.page.cdp.sent])
@@ -272,7 +277,7 @@ class BrowserTest(unittest.TestCase):
         result = self.browser.navigate("https://example.com")
         self.assertTrue(result["ok"])
         self.assertIn(("Page.captureScreenshot",
-                       {"format": "jpeg", "quality": 60}),
+                       {"format": "jpeg", "quality": 50}),
                       [(m, p) for m, p in self.page.cdp.sent])
         frame = self.browser.pump_screencast()
         self.assertEqual(frame, "EAGERDATA")