Align heights of menu/theme buttons, fix button and menu positions

src/serena/resources/dashboard/dashboard.css

@@ -103,8 +103,9 @@ body {
 .header-nav {
     position: relative;
     display: flex;
-    align-items: center;
+    align-items: flex-end;
     width: 200px;
+    height: 100%;
     gap: 10px;
     flex-shrink: 0;
 }
@@ -129,7 +130,7 @@ body {
 
 .menu-dropdown {
     position: absolute;
-    top: 45px;
+    top: calc(var(--header-height) + 10px);
     right: 0;
     background-color: var(--bg-secondary);
     border: 1px solid var(--border-color);

src/serena/resources/dashboard/index.html

@@ -30,7 +30,7 @@
 
     <nav class="header-nav">
         <button id="theme-toggle" class="theme-toggle">
-            <span id="theme-icon">🌙</span>
+            <span id="theme-icon" style="height: 21px">🌙</span>
             <span id="theme-text">Dark</span>
         </button>
         <button id="menu-toggle" class="menu-button">