refactor: use Vite loadEnv instead of manual .env parsing

Replace manual regex-based .env parsing and the unsafe
define: { "process.env": process.env } (which leaked all env vars into
the bundle) with Vite's built-in loadEnv scoped to CANISTER_* prefix.
Also remove unused DFX_NETWORK define (no longer referenced after
removing fetchRootKey).

Author: marc0olo

motoko/hello_world/frontend/vite.config.js

@@ -1,5 +1,4 @@
-import { defineConfig } from "vite";
-import { readFileSync } from "fs";
+import { defineConfig, loadEnv } from "vite";
 import { execSync } from "child_process";
 import { icpBindgen } from "@icp-sdk/bindgen/plugins/vite";
 
@@ -59,15 +58,12 @@ function getDevServerConfig() {
   );
 }
 
-export default defineConfig(({ command }) => {
-  // Load .env file (generated by dfx) if it exists
-  try {
-    const envFile = readFileSync("../.env", "utf-8");
-    for (const line of envFile.split("\n")) {
-      const match = line.match(/^([A-Z_]+[A-Z0-9_]*)=(.+)$/);
-      if (match) process.env[match[1]] = match[2].replace(/^['"]|['"]$/g, "");
-    }
-  } catch {}
+export default defineConfig(({ command, mode }) => {
+  // dfx generates ../.env with CANISTER_ID_* vars on deploy. Bake them into the
+  // bundle so actor.js can fall back to them when the ic_env cookie does not
+  // contain canister IDs (dfx does not inject PUBLIC_CANISTER_ID:* env vars
+  // into the asset canister, unlike icp-cli).
+  const env = loadEnv(mode, "..", ["CANISTER_"]);
 
   return {
     base: "./",
@@ -77,7 +73,11 @@ export default defineConfig(({ command }) => {
         outDir: "./src/bindings",
       }),
     ],
-    define: { "process.env": process.env },
+    define: {
+      "process.env.CANISTER_ID_BACKEND": JSON.stringify(
+        env.CANISTER_ID_BACKEND
+      ),
+    },
     optimizeDeps: {
       esbuildOptions: { define: { global: "globalThis" } },
     },