fix: auto-strip /api/ prefix in api client + document convention

The api client now calls normalizePath() to strip accidental /api/
prefixes. This prevents the recurring /api/api/ double-prefix bug.
Added convention note to .claude/CLAUDE.md so future workers know.

frontend/src/lib/api.ts

@@ -4,6 +4,14 @@ import type { ApiError } from "@/lib/types";
 class ApiClient {
   private baseUrl = "/api";
 
+  /** Strip leading /api/ if accidentally included — baseUrl already provides it */
+  private normalizePath(path: string): string {
+    if (path.startsWith("/api/")) {
+      return path.slice(4); // "/api/foo" -> "/foo"
+    }
+    return path;
+  }
+
   private async getHeaders(): Promise<HeadersInit> {
     const supabase = createClient();
     const {
@@ -29,9 +37,10 @@ class ApiClient {
   }
 
   private async request<T>(
-    path: string,
+    rawPath: string,
     options: RequestInit = {},
   ): Promise<T> {
+    const path = this.normalizePath(rawPath);
     const headers = await this.getHeaders();
     const res = await fetch(`${this.baseUrl}${path}`, {
       ...options,
@@ -80,7 +89,8 @@ class ApiClient {
     return this.request<T>(path, { method: "DELETE" });
   }
 
-  async postFormData<T>(path: string, formData: FormData): Promise<T> {
+  async postFormData<T>(rawPath: string, formData: FormData): Promise<T> {
+    const path = this.normalizePath(rawPath);
     const supabase = createClient();
     const {
       data: { session },