Add Nav, About/Projects pages and nginx hardening

Harden nginx: add CSP, HSTS, Referrer-Policy and Permissions-Policy;
include image/svg+xml in gzip types; set X-Content-Type-Options on
static assets; change try_files to return =404.

Add Nav component and wire into BaseLayout; add About and Projects
pages with projects.json, an initial blog post, and small layout/padding
adjustments (removed redundant back links).

src/pages/blog/[...slug].astro

@@ -27,8 +27,6 @@ function formatDate(date: Date) {
     <main class="main">
         <div class="container">
             <header class="header fade-in">
-                <p class="back"><a href="/blog">← back to blog</a></p>
-                <div class="divider">══════════════════════════════</div>
                 <h1 class="title">{post.data.title}</h1>
                 <p class="date">{formatDate(post.data.date)}</p>
                 <div class="divider">══════════════════════════════</div>
@@ -39,7 +37,6 @@ function formatDate(date: Date) {
             </article>
 
             <footer class="footer fade-in">
-                <div class="divider">══════════════════════════════</div>
                 <p>Made with love by Latte</p>
             </footer>
         </div>
@@ -73,6 +70,7 @@ function formatDate(date: Date) {
         align-items: center;
         justify-content: center;
         padding: var(--space-lg) var(--space-md);
+        padding-top: calc(var(--space-lg) + 3rem);
     }
 
     .container {
@@ -89,19 +87,6 @@ function formatDate(date: Date) {
         margin-bottom: var(--space-lg);
     }
 
-    .back {
-        margin-bottom: var(--space-sm);
-        font-size: 0.85rem;
-    }
-
-    .back a {
-        color: var(--color-text-dim);
-    }
-
-    .back a:hover {
-        color: var(--color-accent);
-    }
-
     .divider {
         color: var(--color-surface);
         text-align: center;
@@ -224,7 +209,7 @@ function formatDate(date: Date) {
     }
 
     .footer {
-        margin-top: var(--space-lg);
+        margin-top: var(--space-xl);
         text-align: center;
     }