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/css/gen.clj

@@ -58,7 +58,11 @@
 (defn base-css
   "Generate base body/reset styles."
   []
-  "body {
+  "*, *::before, *::after {
+  box-sizing: border-box;
+}
+
+body {
   font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
   margin: 0;
   background: var(--bg-0);