feat(web): show version in sidebar footer

lib/volumen/web/views/layout.erb

@@ -93,6 +93,13 @@
         padding: 0.85rem 0.9rem; border-top: 1px solid var(--border);
         display: flex; align-items: center; gap: 0.6rem;
       }
+      .sidebar-version {
+        padding: 0.6rem 0.9rem;
+        border-top: 1px solid var(--border);
+        font-family: var(--font-mono); font-size: 0.68rem;
+        color: var(--fg-subtle); letter-spacing: 0.04em;
+        text-align: center;
+      }
       .avatar {
         width: 30px; height: 30px; border-radius: 50%;
         background: var(--accent-soft); color: var(--accent);
@@ -628,6 +635,7 @@
                 </button>
               </form>
             </div>
+            <div class="sidebar-version">v<%= h(Volumen::VERSION) %></div>
           <% end %>
         </aside>
         <div class="main">