docs(dev): add script and docs for local API key loading

- Create dev-with-env.sh script to load .env before build
- Add npm run dev:tauri command that uses the script
- Document why .env must be loaded before compilation
- Explain compile-time vs runtime environment variables

The issue: option_env!() reads vars at compile time, but .env
loads at runtime. Solution: source .env before cargo build.

cp

package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "scripts": {
     "dev": "vite dev",
+    "dev:tauri": "./scripts/dev-with-env.sh",
     "build": "vite build",
     "preview": "vite preview",
     "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",