refine typography to match website design system

- font-weight 250 with variable font for crisper text
- add letter-spacing 0.3px on body text
- load Inter variable font (100-900 range)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: masnwilliams

pkg/auth/success.html

@@ -2,7 +2,7 @@
 <html>
 <head>
     <title>kernel cli — authenticated</title>
-    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400&display=swap" rel="stylesheet">
+    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@100..900&display=swap" rel="stylesheet">
     <style>
         * {
             margin: 0;
@@ -16,10 +16,11 @@
         }
         body {
             font-family: 'Inter', sans-serif;
-            font-weight: 300;
+            font-weight: 250;
             background-color: #f2f0e7;
             color: #1c2024;
             text-transform: lowercase;
+            letter-spacing: 0.3px;
             -webkit-font-smoothing: antialiased;
             -moz-osx-font-smoothing: grayscale;
             line-height: 1.2;