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/admin/candidatures/[id]/approve/route.ts

@@ -83,7 +83,7 @@ export async function POST(
           const firstname = (candidature as Record<string, unknown>).firstname as string;
 
           await resend.emails.send({
-            from: "HookLab <noreply@hooklab.fr>",
+            from: process.env.RESEND_FROM_EMAIL || "HookLab <onboarding@resend.dev>",
             to: email,
             subject: "Ta candidature HookLab est acceptée !",
             html: `

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: `

app/api/stripe/webhook/route.ts

@@ -126,7 +126,7 @@ 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: email,
               subject: "Bienvenue dans HookLab ! Tes accès sont prêts",
               html: `