feat: swap Merriweather → Inter + define @layer base

- Remove Merriweather font import and config from layout.tsx
- Add Inter with weight/style/adjustFontFallback
- Apply inter.className to body instead of font-serif
- Update --font-serif in @theme to use Inter
- Add @layer base with body, h1-h6, code, and link rules

app/globals.css

@@ -15,7 +15,7 @@
 
   /* Fonts — override defaults */
   --font-sans: "Inter", ui-sans-serif, system-ui, sans-serif;
-  --font-serif: "Merriweather", ui-serif, Georgia, serif;
+  --font-serif: 'Inter', -apple-system, system-ui, sans-serif;
   --font-mono: "JetBrains Mono", ui-monospace, monospace;
 
   /* Typography scale */
@@ -51,6 +51,36 @@
   }
 }
 
+@layer base {
+  body {
+    font-family: var(--font-serif);
+    font-weight: 400;
+    line-height: 1.6;
+    -webkit-font-smoothing: antialiased;
+    -moz-osx-font-smoothing: grayscale;
+  }
+  h1, h2, h3, h4, h5, h6 {
+    font-family: var(--font-sans);
+    font-weight: 700;
+    line-height: 1.25;
+    letter-spacing: -0.015em;
+  }
+  h1 { line-height: 1.15; letter-spacing: -0.025em; }
+  h2 { line-height: 1.2; letter-spacing: -0.02em; }
+  code, pre, kbd, samp {
+    font-family: var(--font-mono);
+    font-feature-settings: 'liga' 1, 'calt' 1;
+  }
+  a {
+    text-underline-offset: 3px;
+    text-decoration-thickness: 1px;
+    transition: text-decoration-thickness 0.15s ease;
+  }
+  a:hover {
+    text-decoration-thickness: 3px;
+  }
+}
+
 /* === Dark mode token overrides === */
 .dark {
   --color-canvas: oklch(0.12 0.02 240);