fix(ui): enforce accessible theme contrast

src/test/java/com/lab/labtimesheet/ui/UiContractWebTest.java

@@ -6,6 +6,8 @@ import static org.springframework.test.web.servlet.request.MockMvcRequestBuilder
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
 
 import java.nio.charset.StandardCharsets;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 import org.junit.jupiter.api.Test;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
@@ -70,6 +72,28 @@ class UiContractWebTest {
         assertTrue(themeBootstrap.contains("matchMedia('(prefers-color-scheme: dark)')"));
     }
 
+    @Test
+    void themeTokensMeetTextFocusAndMeaningfulBoundaryContrast() throws Exception {
+        String css = new ClassPathResource("static/assets/app.css")
+                .getContentAsString(StandardCharsets.UTF_8);
+        String light = section(css, ":root\\{color-scheme:light;([^}]*)}");
+        String dark = section(css, ":root\\[data-theme=dark]\\{color-scheme:dark;([^}]*)}");
+
+        assertContrast(light, "ink", "canvas", 4.5);
+        assertContrast(light, "muted", "panel", 4.5);
+        assertContrast(light, "subtle", "sidebar", 4.5);
+        assertContrast(light, "border", "canvas", 3.0);
+        assertContrast(light, "border-strong", "panel", 3.0);
+        assertContrast(light, "focus", "canvas", 3.0);
+
+        assertContrast(dark, "ink", "canvas", 4.5);
+        assertContrast(dark, "muted", "panel", 4.5);
+        assertContrast(dark, "subtle", "sidebar", 4.5);
+        assertContrast(dark, "border", "panel", 3.0);
+        assertContrast(dark, "border-strong", "panel", 3.0);
+        assertContrast(dark, "focus", "panel", 3.0);
+    }
+
     @Test
     @WithMockUser(username = "admin@example.test", roles = "ADMIN")
     void sharedComponentsExposeAccessibleFormsStatusAndEmptyState() throws Exception {
@@ -100,4 +124,48 @@ class UiContractWebTest {
             return "test/components-consumer";
         }
     }
+
+    private static String section(String css, String expression) {
+        Matcher matcher = Pattern.compile(expression).matcher(css);
+        assertTrue(matcher.find(), () -> "Missing CSS token section: " + expression);
+        return matcher.group(1);
+    }
+
+    private static void assertContrast(String section, String foreground, String background, double minimum) {
+        double ratio = contrast(color(section, foreground), color(section, background));
+        assertTrue(ratio >= minimum,
+                () -> foreground + " / " + background + " contrast " + ratio + " is below " + minimum);
+    }
+
+    private static String color(String section, String name) {
+        Matcher matcher = Pattern.compile("--" + Pattern.quote(name) + ":(#[0-9a-fA-F]{3}(?:[0-9a-fA-F]{3})?)")
+                .matcher(section);
+        assertTrue(matcher.find(), () -> "Missing CSS color token: " + name);
+        String value = matcher.group(1);
+        if (value.length() == 4) {
+            return "#" + value.charAt(1) + value.charAt(1)
+                    + value.charAt(2) + value.charAt(2)
+                    + value.charAt(3) + value.charAt(3);
+        }
+        return value;
+    }
+
+    private static double contrast(String first, String second) {
+        double lighter = Math.max(luminance(first), luminance(second));
+        double darker = Math.min(luminance(first), luminance(second));
+        return (lighter + 0.05) / (darker + 0.05);
+    }
+
+    private static double luminance(String hex) {
+        double red = linear(Integer.parseInt(hex.substring(1, 3), 16) / 255.0);
+        double green = linear(Integer.parseInt(hex.substring(3, 5), 16) / 255.0);
+        double blue = linear(Integer.parseInt(hex.substring(5, 7), 16) / 255.0);
+        return 0.2126 * red + 0.7152 * green + 0.0722 * blue;
+    }
+
+    private static double linear(double component) {
+        return component <= 0.04045
+                ? component / 12.92
+                : Math.pow((component + 0.055) / 1.055, 2.4);
+    }
 }