feat: add demo link and GitHub icon to the toolbar

- README: link to the GitHub Pages demo
- Toolbar: GitHub icon below the theme button linking to the repo

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Author: kvaps

README.md

@@ -6,6 +6,8 @@
 
 Markdown editor with a live mindmap preview. Edit on either side — they stay in sync.
 
+**[Live demo →](https://kvaps.github.io/stackmind/)**
+
 <picture>
   <source media="(prefers-color-scheme: dark)" srcset="docs/screenshot-dark.png">
   <source media="(prefers-color-scheme: light)" srcset="docs/screenshot-light.png">

src/components/Toolbar.vue

@@ -66,6 +66,18 @@ function cycleTheme() {
           <path d="M12 3 A9 9 0 0 0 12 21 Z" fill="currentColor" stroke="none" />
         </svg>
       </button>
+      <a
+        class="toolbar__btn"
+        href="https://github.com/kvaps/stackmind"
+        target="_blank"
+        rel="noopener noreferrer"
+        title="View on GitHub"
+        aria-label="View on GitHub"
+      >
+        <svg viewBox="0 0 24 24" width="20" height="20" aria-hidden="true" fill="currentColor">
+          <path d="M12 .5C5.65.5.5 5.65.5 12c0 5.08 3.29 9.39 7.86 10.91.57.1.78-.25.78-.55v-2.13c-3.2.7-3.87-1.36-3.87-1.36-.52-1.34-1.27-1.69-1.27-1.69-1.04-.71.08-.7.08-.7 1.15.08 1.76 1.18 1.76 1.18 1.02 1.76 2.68 1.25 3.34.96.1-.74.4-1.25.72-1.54-2.55-.29-5.23-1.28-5.23-5.7 0-1.26.45-2.29 1.18-3.1-.12-.29-.51-1.46.11-3.04 0 0 .97-.31 3.18 1.18.92-.26 1.91-.39 2.9-.39s1.98.13 2.9.39c2.21-1.49 3.18-1.18 3.18-1.18.62 1.58.23 2.75.11 3.04.74.81 1.18 1.84 1.18 3.1 0 4.43-2.69 5.41-5.25 5.69.41.36.78 1.07.78 2.16v3.21c0 .31.21.66.79.55C20.21 21.38 23.5 17.08 23.5 12 23.5 5.65 18.35.5 12 .5z"/>
+        </svg>
+      </a>
     </div>
   </nav>
 </template>
@@ -98,6 +110,7 @@ function cycleTheme() {
   color: var(--toolbar-icon, #555);
   border-radius: 4px;
   cursor: pointer;
+  text-decoration: none;
   transition: background 120ms, color 120ms;
 }
 .toolbar__btn:hover {