fix(reports): scope global spinner wait to non-tiled path only

The pre-branch viewport-only wait_for_function ran regardless of which
screenshot path was taken. For tiled screenshots, per-tile checks in
take_tiled_screenshot already handle every viewport as we scroll, so
the pre-branch check is redundant there. For non-tiled screenshots,
element.screenshot() captures the full element including below-the-fold
content, so a global (all-spinners) check is correct.

Move the global querySelector('.loading').length === 0 check into each
non-tiled branch and remove it from the pre-branch position, so:
- non-tiled path: global check (waits for all charts including below fold)
- tiled path: no pre-check; per-tile viewport checks in screenshot_utils
  handle spinner detection tile by tile

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

tests/unit_tests/utils/webdriver_test.py

@@ -684,16 +684,7 @@ class TestWebDriverPlaywrightErrorHandling:
             driver.get_screenshot("http://example.com", "test-element", mock_user)
 
         mock_page.wait_for_function.assert_called_once_with(
-            """() => {
-                            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;
-                        }""",
+            "() => document.querySelectorAll('.loading').length === 0",
             timeout=60 * 1000,
         )
         # Guard against reintroducing the old snapshot-based approach