Fix apiFetch basePath: use publicRuntimeConfig instead of NEXT_PUBLIC_ env var

NEXT_PUBLIC_* vars are inlined at build time — not available in Docker build context.
publicRuntimeConfig is resolved at runtime from next.config.mjs, which works correctly.

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

frontend/lib/apiFetch.ts

@@ -2,9 +2,13 @@
  * Wrapper around fetch that prepends the Next.js basePath so API calls
  * reach the correct backend when deployed under a sub-path (e.g. /ppt-tool).
  *
+ * basePath is read from next.config.mjs at build time via next/config.
  * Usage: apiFetch('/api/v1/...', options)  — identical to fetch(), just works.
  */
-const BASE_PATH = process.env.NEXT_PUBLIC_BASE_PATH ?? '';
+import getConfig from 'next/config';
+
+const { publicRuntimeConfig } = getConfig() ?? {};
+const BASE_PATH: string = publicRuntimeConfig?.basePath ?? '';
 
 export function apiFetch(path: string, init?: RequestInit): Promise<Response> {
   const url = path.startsWith('/api/') ? `${BASE_PATH}${path}` : path;

frontend/next.config.mjs

@@ -4,6 +4,9 @@ const API_URL = process.env.API_INTERNAL_URL || 'http://localhost:8000';
 const nextConfig = {
   basePath: "/ppt-tool",
   assetPrefix: "/ppt-tool",
+  publicRuntimeConfig: {
+    basePath: "/ppt-tool",
+  },
   reactStrictMode: false,
   distDir: ".next-build",