Fix: Base path resolution timing issue for subpath deployments (#572)

When deployed under a URL prefix (e.g., /shelfmark), images loaded by
React were not respecting the base path, causing 404 errors. The logo
would incorrectly load from /logo.png instead of /shelfmark/logo.png.

The root cause was that the BASE_PATH constant was being initialized at
module load time, before the DOM was fully parsed. This meant
document.querySelector('base') returned null, causing BASE_PATH to
default to '/' regardless of the actual base tag value.

Changed to lazy initialization pattern where the base path is resolved
on first access, ensuring the DOM and base tag are ready.

Fixes [#571](https://github.com/calibrain/shelfmark/issues/571)

src/frontend/src/utils/basePath.ts

@@ -30,16 +30,23 @@ const resolveBasePath = (): string => {
   }
 };
 
-const BASE_PATH = normalizeBasePath(resolveBasePath());
+// Lazy initialization to ensure DOM is ready when base path is resolved
+let _basePath: string | null = null;
 
-export const getBasePath = (): string => BASE_PATH;
+export const getBasePath = (): string => {
+  if (_basePath === null) {
+    _basePath = normalizeBasePath(resolveBasePath());
+  }
+  return _basePath;
+};
 
 export const withBasePath = (path: string): string => {
+  const basePath = getBasePath();
   const normalizedPath = path.startsWith('/') ? path : `/${path}`;
-  if (BASE_PATH === '/') {
+  if (basePath === '/') {
     return normalizedPath;
   }
-  return `${BASE_PATH}${normalizedPath}`;
+  return `${basePath}${normalizedPath}`;
 };
 
 export const getApiBase = (): string => withBasePath('/api');