feat: wire Sentry, conditional on DSN env var

apps/web (@sentry/nextjs):
- sentry.client.config.ts / sentry.server.config.ts / sentry.edge.config.ts
  initialize Sentry only when NEXT_PUBLIC_SENTRY_DSN (or SENTRY_DSN on the
  server) is set; tracesSampleRate 0.1
- next.config.mjs wraps the existing config with withSentryConfig only when a
  DSN is present; source-map upload stays disabled until SENTRY_AUTH_TOKEN is
  provided
- .env.example documents NEXT_PUBLIC_SENTRY_DSN and the optional auth token

apps/api (@sentry/node):
- src/lib/sentry.ts initializes Sentry at module load when SENTRY_DSN is set
- server.ts imports sentry.ts as the very first side-effect so early-boot
  errors (env validation, plugin registration) reach Sentry
- env schema gains optional SENTRY_DSN (URL)
- app.ts registers an errorHandler that captures the exception with the
  authenticated user when SENTRY_DSN is set; logs and re-sends as before

Build-time housekeeping:
- profile route: targetDate ?? null on create to satisfy Prisma's input shape
  under exactOptionalPropertyTypes
- profile form: useForm receives defaultValues only when initial is provided
  (spread instead of `defaultValues: undefined`); Field error prop typed
  `string | undefined` for exactOptionalPropertyTypes

Without a DSN both apps run unchanged (Sentry is inert).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/app/[locale]/profile/profile-form.tsx

@@ -28,17 +28,19 @@ export function ProfileForm({ initial }: { initial: Profile | null }) {
     formState: { errors, isSubmitting },
   } = useForm<ProfileInput>({
     resolver: zodResolver(profileInputSchema),
-    defaultValues: initial
+    ...(initial
       ? {
-          age: initial.age,
-          gender: initial.gender,
-          heightCm: initial.heightCm,
-          weightStartKg: initial.weightStartKg,
-          weightCurrentKg: initial.weightCurrentKg,
-          weightGoalKg: initial.weightGoalKg,
-          activityLevel: initial.activityLevel,
+          defaultValues: {
+            age: initial.age,
+            gender: initial.gender,
+            heightCm: initial.heightCm,
+            weightStartKg: initial.weightStartKg,
+            weightCurrentKg: initial.weightCurrentKg,
+            weightGoalKg: initial.weightGoalKg,
+            activityLevel: initial.activityLevel,
+          },
         }
-      : undefined,
+      : {}),
   });
 
   const onSubmit = handleSubmit(async (input) => {
@@ -142,7 +144,7 @@ function Field({
   children,
 }: {
   label: string;
-  error?: string;
+  error?: string | undefined;
   children: React.ReactNode;
 }) {
   return (