docs: add Web UI screenshot to README (#184)

Add screenshot of the SnapDiff Web UI report showing side-by-side
baseline vs current comparison. This was the #1 finding across 6
rounds of brutal honesty review — a visual testing gem needs visuals
in its README.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

README.md

@@ -109,7 +109,11 @@ Add one line to get an interactive dashboard for reviewing all screenshot differ
 require 'capybara_screenshot_diff/reporters/html'
 ```
 
-After tests run, open `doc/screenshots/snap_diff_report.html` — side-by-side comparison with 4 view modes (both/base/new/heatmap), per-image zoom, annotation toggle, keyboard navigation, and search.
+After tests run, open `doc/screenshots/snap_diff_report.html`:
+
+![SnapDiff Web UI — side-by-side comparison with baseline and current screenshots](docs/images/snap_diff_web_ui.png)
+
+4 view modes (both/base/new/heatmap), per-image zoom, annotation toggle, keyboard navigation, and search.
 
 **In GitHub Actions**, the report renders inline as a CI artifact — no download needed. Add a PR comment with a link to the report automatically: