feat ui foundation layer for page container sections and grid

src/styles.scss

@@ -49,6 +49,11 @@
   padding: 0;
 }
 
+*::before,
+*::after {
+  box-sizing: inherit;
+}
+
 html {
   font-size: 16px;
   -webkit-font-smoothing: antialiased;
@@ -57,7 +62,7 @@ html {
 }
 
 body {
-  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
+  font-family: 'DM Sans', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Helvetica Neue', Arial, sans-serif;
   line-height: 1.6;
   color: var(--text-primary);
   background: #f5f5f5;
@@ -69,6 +74,23 @@ body {
   }
 }
 
+h1, h2, h3, h4, h5, h6 {
+  font-weight: 700;
+  line-height: 1.25;
+}
+
+p {
+  line-height: 1.5;
+}
+
+img,
+svg,
+video,
+canvas {
+  display: block;
+  max-width: 100%;
+}
+
 /* Smooth Transitions */
 a, button, input, textarea, select {
   transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
@@ -109,10 +131,61 @@ a, button, input, textarea, select {
 }
 
 /* Utility Classes */
-.container {
-  max-width: 1200px;
+.page-container {
+  max-width: 1280px;
   margin: 0 auto;
-  padding: 0 20px;
+  padding: 24px 16px;
+  width: 100%;
+}
+
+/* Backward-compatible alias while foundation migration is in progress. */
+.container {
+  max-width: 1280px;
+  margin: 0 auto;
+  padding: 24px 16px;
+  width: 100%;
+}
+
+.section {
+  margin: 24px 0;
+}
+
+.grid {
+  display: grid;
+  gap: 16px;
+}
+
+.grid-2 {
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+}
+
+.grid-3 {
+  grid-template-columns: repeat(3, minmax(0, 1fr));
+}
+
+.grid-4 {
+  grid-template-columns: repeat(4, minmax(0, 1fr));
+}
+
+@media (max-width: 1200px) {
+  .grid-4 {
+    grid-template-columns: repeat(3, minmax(0, 1fr));
+  }
+}
+
+@media (max-width: 900px) {
+  .grid-3,
+  .grid-4 {
+    grid-template-columns: repeat(2, minmax(0, 1fr));
+  }
+}
+
+@media (max-width: 640px) {
+  .grid-2,
+  .grid-3,
+  .grid-4 {
+    grid-template-columns: 1fr;
+  }
 }
 
 .text-center {