fix: improve CSS layout with box-sizing reset, responsive alerts, and form spacing

- Add universal box-sizing: border-box reset to base styles
- Make alert component responsive with container queries (flex-wrap,
  full-width title/body at narrow widths)
- Increase form legend margin-bottom from --size-1 to --size-3

src/ui/alert.css

@@ -1,6 +1,8 @@
 .alert {
+  container-type: inline-size;
   position: relative;
   display: flex;
+  flex-wrap: wrap;
   align-items: center;
   gap: var(--size-3);
   padding: var(--size-4) var(--size-6);
@@ -19,6 +21,13 @@
   margin: 0;
 }
 
+@container (max-width: 24rem) {
+  .alert-title,
+  .alert-body {
+    flex-basis: 100%;
+  }
+}
+
 .alert-success {
   border: none;
   color: var(--success);