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>

docs/auth/desktop-integration.md

@@ -68,7 +68,7 @@ pnpm dev:web
 pnpm dev:desktop
 ```
 
-本地调试时，Next.js rewrites（配置在 `apps/web/next.config.ts`）自动将 `/api/*` 请求代理到 `api-dev.copilothub.ai`。
+本地调试时，Next.js rewrites（配置在 `apps/web/next.config.ts`）自动将 `/api/*` 请求代理到 `MULTICA_API_URL` 指定的后端。
 
 ## 参考
 

docs/development.md

@@ -51,7 +51,7 @@ pnpm dev:local:archive    # Archive dev data and start fresh
 
 | Variable | Description |
 |----------|-------------|
-| `API_URL` | Backend API URL (default: `https://api-dev.copilothub.ai`) |
+| `MULTICA_API_URL` | Backend API URL (required, no default) |
 
 **Build for different environments:**
 
@@ -61,7 +61,7 @@ pnpm --filter @multica/desktop build              # Production (.env.production)
 pnpm --filter @multica/desktop build:staging      # Staging (.env.staging)
 
 # Web (Vercel)
-# Set API_URL in Vercel Dashboard → Settings → Environment Variables
+# Set MULTICA_API_URL in Vercel Dashboard → Settings → Environment Variables
 ```
 
 See `apps/desktop/.env.example` for the full variable reference.