refactor: unify API URL env var to MULTICA_API_URL

Replace scattered API_URL, MAIN_VITE_API_URL, and RENDERER_VITE_API_URL
with a single MULTICA_API_URL across all apps and packages.

- Desktop: use envPrefix to expose MULTICA_* to main process, rename
  RENDERER_VITE_API_URL → RENDERER_VITE_MULTICA_API_URL, remove
  MAIN_VITE_API_URL (now read directly via MULTICA_API_URL)
- Web: add .env.development with MULTICA_API_URL, enforce required check
  in next.config.ts, update .gitignore to allow .env.development
- Core: make MULTICA_API_URL required in api-client (no silent fallback)
- Scripts: pass MULTICA_API_URL in dev-local.sh for web process
- Turbo: update globalEnv from API_URL to MULTICA_API_URL
- Docs: update references to the new env var name

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

apps/web/next.config.ts

@@ -1,11 +1,15 @@
 import type { NextConfig } from "next";
 
+if (!process.env.MULTICA_API_URL) {
+  throw new Error("MULTICA_API_URL is required");
+}
+
 const nextConfig: NextConfig = {
   transpilePackages: ["@multica/ui", "@multica/store", "@multica/hooks", "@multica/sdk"],
   rewrites: async () => [
     {
       source: "/api/:path*",
-      destination: `${process.env.API_URL || "https://api.multica.ai"}/api/:path*`,
+      destination: `${process.env.MULTICA_API_URL}/api/:path*`,
     },
   ],
   headers: async () => [