docs: add VISUAL_TESTING.md for visual regression baseline management

Document the hybrid visual testing stack (Storybook + Chromatic for
components, Playwright screenshot assertions for E2E flows) so teams have a
clear process for accepting intentional changes vs. catching regressions.

Covers how the two layers work, accepting changes (Chromatic UI and local
Playwright baseline updates), common scenarios, troubleshooting, the team
PR workflow, configuration files, and determinism conventions. Linked from
the README Testing section.

Author: Task Orchestrator Agent

README.md

@@ -262,6 +262,7 @@ cd electron && npx playwright install chromium && npm run test:e2e
 ```
 
 For detailed testing documentation, see [web/TESTING.md](web/TESTING.md).
+For visual regression testing and baseline management, see [VISUAL_TESTING.md](VISUAL_TESTING.md).
 
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