fix: make API calls respect BASE_PATH for subpath deployments

config.apiBaseUrl now derives from <base> tag when no explicit config
is set (e.g., /server/api/v1 instead of /api/v1). commands.ts authFetch
prepends apiBaseUrl and uses relative paths.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/config.ts

@@ -6,8 +6,14 @@ declare global {
   }
 }
 
+/** Base path from <base href="..."> tag, or '/' if none. Always ends with '/'. */
+const basePath = document.querySelector('base')?.getAttribute('href') ?? '/';
+
 export const config = {
   get apiBaseUrl(): string {
-    return window.__CAMELEER_CONFIG__?.apiBaseUrl ?? '/api/v1';
+    return window.__CAMELEER_CONFIG__?.apiBaseUrl ?? `${basePath}api/v1`;
+  },
+  get basePath(): string {
+    return basePath;
   },
 };