style: icon-only moon/sun theme toggle in header corner

Author: systemslibrarian

src/ui.ts

@@ -134,7 +134,7 @@ function installThemeToggle(): void {
   const syncLabel = (): void => {
     const isDark = root.dataset.theme !== "light";
     if (button) {
-      button.textContent = isDark ? "☀ Light Mode" : "☾ Dark Mode";
+      button.textContent = isDark ? "🌙" : "☀️";
       button.setAttribute("aria-label", isDark ? "Switch to light mode" : "Switch to dark mode");
     }
   };
@@ -306,7 +306,7 @@ function template(): string {
       <header class="hero" role="banner">
         <div class="chip-row" role="list" aria-label="Category and controls">
           <span class="chip category" role="listitem">Format-Preserving Encryption</span>
-          <button id="theme-toggle" class="chip action" type="button" aria-label="Switch color theme">Switch to Light</button>
+          <button id="theme-toggle" class="theme-toggle" type="button" aria-label="Switch color theme"></button>
         </div>
         <h1>Format Ward</h1>
         <p class="subtitle">Interactive FF1 and FF3-1 demo over real WebCrypto AES rounds from NIST SP 800-38G.</p>

styles/main.css

@@ -123,6 +123,7 @@ body {
 
 /* ── Hero ── */
 .hero {
+  position: relative;
   background: linear-gradient(145deg, var(--surface), var(--surface-2));
   border: 1px solid var(--border);
   border-radius: 20px;
@@ -181,6 +182,30 @@ h1 {
   justify-content: center;
 }
 
+/* ── Theme toggle (icon button) ── */
+.theme-toggle {
+  position: absolute;
+  top: 0;
+  right: 0;
+  width: 2.6rem;
+  height: 2.6rem;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  border: 1px solid var(--border);
+  border-radius: 50%;
+  background: color-mix(in srgb, var(--surface-2) 70%, transparent);
+  color: var(--text);
+  font-size: 1.25rem;
+  cursor: pointer;
+  line-height: 1;
+  padding: 0;
+  transition: background 0.15s ease;
+}
+.theme-toggle:hover {
+  background: color-mix(in srgb, var(--accent) 25%, transparent);
+}
+
 /* ── Sections ── */
 .why,
 .refs,