refactor: remove exact cross-layer css duplicates

tests/e2e/app-shell.spec.js

@@ -1803,6 +1803,30 @@ test("heart breathing prepares once then contracts on each exhale", async ({ pag
   await expect(page.locator(".heart-breath-ripple span").first()).toHaveCSS("transition-duration", "4s");
 });
 
+test("stylesheet layers do not repeat identical top-level rules", async ({ page }) => {
+  await mockApplication(page, session("admin", true));
+  await page.goto("/index.html");
+
+  const duplicates = await page.evaluate(() => {
+    const occurrences = new Map();
+    for (const sheet of Array.from(document.styleSheets)) {
+      const href = sheet.href ? new URL(sheet.href).pathname : "inline";
+      for (const rule of Array.from(sheet.cssRules || [])) {
+        if (typeof rule.selectorText !== "string" || !rule.style) continue;
+        const key = `${rule.selectorText}\u0000${rule.style.cssText}`;
+        const rows = occurrences.get(key) || [];
+        rows.push(href);
+        occurrences.set(key, rows);
+      }
+    }
+    return Array.from(occurrences.entries())
+      .filter(([, paths]) => new Set(paths).size > 1)
+      .map(([rule, paths]) => ({ rule, paths }));
+  });
+
+  expect(duplicates).toEqual([]);
+});
+
 test("mobile shell stays within the viewport", async ({ page }) => {
   await page.setViewportSize({ width: 375, height: 812 });
   await mockApplication(page, session("user", true));