fix: use configurable RESEND_FROM_EMAIL with onboarding@resend.dev fallback

Allow sending emails without custom domain by defaulting to Resend's
free onboarding address. Set RESEND_FROM_EMAIL env var later when
hooklab.fr domain is purchased and verified.

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

app/api/candidature/route.ts

@@ -114,7 +114,7 @@ export async function POST(request: Request) {
         const resend = new Resend(process.env.RESEND_API_KEY);
 
         await resend.emails.send({
-          from: "HookLab <noreply@hooklab.fr>",
+          from: process.env.RESEND_FROM_EMAIL || "HookLab <onboarding@resend.dev>",
           to: body.email,
           subject: "Candidature HookLab reçue !",
           html: `