fix(js): patch fetch to prepend ROOT_PATH for IIS reverse proxy

All relative /api/... calls automatically get /gomag prefix via
global fetch wrapper in shared.js. ROOT_PATH injected from template.

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

api/app/static/js/shared.js

@@ -1,5 +1,16 @@
 // shared.js - Unified utilities for all pages
 
+// ── Root path patch — prepend ROOT_PATH to all relative fetch calls ───────
+(function() {
+    const _fetch = window.fetch.bind(window);
+    window.fetch = function(url, ...args) {
+        if (typeof url === 'string' && url.startsWith('/') && window.ROOT_PATH) {
+            url = window.ROOT_PATH + url;
+        }
+        return _fetch(url, ...args);
+    };
+})();
+
 // ── HTML escaping ─────────────────────────────────
 function esc(s) {
     if (s == null) return '';