Fix font buffer slice and env var access for production SSR

Node.js Buffer pool shares an ArrayBuffer — slice to give Satori's
OpenType parser a standalone copy starting at byte 0. Use process.env
instead of import.meta.env for BACKEND_INTERNAL_URL since Vite only
exposes PUBLIC_* prefixed vars via import.meta.env.

frontend/src/lib/og-renderer.tsx

@@ -17,7 +17,10 @@ function loadFont(): ArrayBuffer {
   ];
   for (const path of candidates) {
     try {
-      return readFileSync(path).buffer as ArrayBuffer;
+      const buf = readFileSync(path);
+      // Node.js Buffers share a pooled ArrayBuffer — slice to get a
+      // standalone copy so Satori's OpenType parser starts at byte 0.
+      return buf.buffer.slice(buf.byteOffset, buf.byteOffset + buf.byteLength);
     } catch {
       // try next
     }

frontend/src/lib/server-api.ts

@@ -1,5 +1,7 @@
+// process.env is required here — Vite's import.meta.env only exposes
+// PUBLIC_* prefixed vars. This module runs server-side only (SSR).
 export const INTERNAL_API_BASE =
-  import.meta.env.BACKEND_INTERNAL_URL || 'http://localhost:8099';
+  process.env.BACKEND_INTERNAL_URL || 'http://localhost:8099';
 
 export async function fetchNotebookMeta(id: string) {
   const url = `${INTERNAL_API_BASE}/api/notebooks/${encodeURIComponent(id)}`;