fix: ensure E2E screenshot tests work consistently across CI and local (#150)

- Remove platform-specific browser channel (was using Chrome on macOS,
  Chromium in CI) - now uses default Chromium everywhere
- Add Docker-based test scripts for CI-identical local testing:
  - `npm run test:e2e:docker` - run tests in Docker
  - `npm run test:e2e:docker:update` - regenerate golden screenshots
- Regenerate golden screenshots using Docker/Linux environment

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

playwright.config.ts

@@ -21,10 +21,8 @@ export default defineConfig({
       name: "chromium",
       use: {
         ...devices["Desktop Chrome"],
-        launchOptions: {
-          // Use system Chrome on macOS for stability, default chromium in CI
-          ...(process.platform === "darwin" ? { channel: "chrome" } : {}),
-        },
+        // Use default Chromium everywhere for consistent screenshot rendering
+        // Run `npm run test:e2e:docker` locally for CI-identical results
       },
     },
   ],