fix(reports): narrow spinner checks to viewport and tighten exception handling

- Replace global '.loading' count check in webdriver.py with a
  getBoundingClientRect viewport-visibility check to avoid deadlock
  when DashboardVirtualization renders off-screen placeholder spinners
- Narrow except clause in screenshot_utils.py from bare Exception to
  PlaywrightTimeout so non-timeout errors (e.g. browser crash) propagate
- Fix load_wait default from 30 s to 60 s to match SCREENSHOT_LOAD_WAIT
  config default
- Add tests covering per-tile spinner wait, timeout warning, non-timeout
  propagation, and load_wait default value

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit_tests/utils/webdriver_test.py

@@ -684,7 +684,16 @@ class TestWebDriverPlaywrightErrorHandling:
             driver.get_screenshot("http://example.com", "test-element", mock_user)
 
         mock_page.wait_for_function.assert_called_once_with(
-            "() => document.querySelectorAll('.loading').length === 0",
+            """() => {
+                            const els = document.querySelectorAll('.loading');
+                            for (const el of els) {
+                                const r = el.getBoundingClientRect();
+                                if (r.top < window.innerHeight && r.bottom > 0) {
+                                    return false;
+                                }
+                            }
+                            return true;
+                        }""",
             timeout=60 * 1000,
         )
         # Guard against reintroducing the old snapshot-based approach