Add dropdown menu styles and constrain image width

Add CSS for the projects dropdown menu (desktop and mobile) and
limit images in main content to max-width: 100%.

static/style.css

@@ -155,6 +155,54 @@ header a:hover:not(.home) {
   gap: 1rem;
 }
 
+.dropdown {
+  position: relative;
+}
+
+.dropdown-toggle {
+  cursor: default;
+  color: var(--fg-color);
+  border-bottom: 2px solid transparent;
+  padding-bottom: 0.1em;
+}
+
+.dropdown:hover .dropdown-toggle {
+  border-bottom: 2px solid var(--link-text-color);
+}
+
+.dropdown-menu {
+  display: none;
+  position: absolute;
+  top: calc(100% + 0.75rem);
+  left: 50%;
+  transform: translateX(-50%);
+  background-color: var(--bg-color);
+  border: 1px solid var(--header-border-color);
+  border-radius: 0.3em;
+  padding: 0.4em 0;
+  z-index: 100;
+  min-width: 10rem;
+  flex-direction: column;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
+}
+
+.dropdown:hover .dropdown-menu {
+  display: flex;
+}
+
+.dropdown-menu a {
+  padding: 0.4em 1.2em;
+  color: var(--fg-color);
+  border-bottom: none !important;
+  white-space: nowrap;
+}
+
+.dropdown-menu a:hover {
+  color: var(--link-text-color);
+  background-color: var(--highlight-bg-color);
+  border-bottom: none !important;
+}
+
 header .icon {
   display: flex;
   align-items: center;
@@ -199,6 +247,19 @@ header .icon {
   header:not(.open) .navigation-block {
     display: none;
   }
+
+  .dropdown-menu {
+    position: static;
+    transform: none;
+    display: flex;
+    box-shadow: none;
+    border: none;
+    padding: 0 0 0 1em;
+  }
+
+  .dropdown-toggle {
+    display: none;
+  }
 }
 
 /* HERO */
@@ -416,6 +477,11 @@ blockquote {
   padding-left: 1em;
 }
 
+main img {
+  max-width: 100%;
+  height: auto;
+}
+
 .links {
   display: flex;
   flex-wrap: wrap;