Do not delay a browser restart without a screen; clearer reload error

Without a connected monitor every Chromium start waited 30 s for one, so
DevTools was unreachable for ~35 s after a restart. Only wait right after
boot. The reload error now says Chromium may still be starting.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_browser.py

@@ -78,3 +78,17 @@ def test_local_pages_exist():
     www = Path(__file__).resolve().parents[1] / "www"
     for page in ("start.html", "offline.html", "setup.html", "kiosk.css", "logo.svg"):
         assert (www / page).exists()
+
+
+def test_prepare_display_waits_for_a_screen_only_after_boot(paths, monkeypatch):
+    calls = []
+    monkeypatch.setattr(b, "_run", lambda args: calls.append(args) or None)
+    monkeypatch.setattr(b.time, "sleep", lambda s: None)
+    monkeypatch.setattr(b.sysinfo, "uptime_seconds", lambda: 3600.0)
+    assert b.prepare_display(paths, dict(DEFAULTS)) is None
+    assert sum(c == ["xrandr", "--query"] for c in calls) == 1
+
+    calls.clear()
+    monkeypatch.setattr(b.sysinfo, "uptime_seconds", lambda: 40.0)
+    b.prepare_display(paths, dict(DEFAULTS))
+    assert sum(c == ["xrandr", "--query"] for c in calls) == 15