Fix onboarding bottom button not staying pinned, and swipe card crash

- OnboardingScreen used min-h-dvh instead of h-dvh, so the scrollable content
  area never actually clipped/scrolled internally; the page just grew and the
  primary CTA button scrolled away with it instead of staying visible. Switch
  to h-dvh and give each step's button a sticky footer (StepFooterButton).
- SwipeCard crashed with "Cannot read properties of null" when a candidate
  had no photos, because the backend could return null instead of an empty
  array; guard with optional chaining.

src/components/onboarding/OnboardingScreen.tsx

@@ -30,8 +30,8 @@ export function OnboardingScreen({
   const total = ONBOARDING_STEPS.length;
 
   return (
-    <div className="flex flex-col flex-1 min-h-dvh">
-      <div className="flex items-center gap-3 px-5 pt-[calc(env(safe-area-inset-top)+16px)] pb-2">
+    <div className="flex flex-col h-dvh">
+      <div className="shrink-0 flex items-center gap-3 px-5 pt-[calc(env(safe-area-inset-top)+16px)] pb-2">
         <button
           onClick={onBack ?? (() => router.back())}
           className="w-9 h-9 rounded-full flex items-center justify-center hover:bg-(--color-bg-secondary) shrink-0"
@@ -65,7 +65,7 @@ export function OnboardingScreen({
         </span>
       </div>
 
-      <div className="flex flex-col gap-2 px-6 pt-6 pb-2">
+      <div className="shrink-0 flex flex-col gap-2 px-6 pt-6 pb-2">
         <h1 className="font-[var(--font-display)] font-extrabold text-[24px] text-(--color-content-primary) leading-snug">
           {title} <span>{emoji}</span>
         </h1>

src/components/onboarding/StepFooterButton.tsx

@@ -0,0 +1,25 @@
+"use client";
+
+import { ButtonHTMLAttributes } from "react";
+import { Button } from "@/components/ui/Button";
+
+interface StepFooterButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
+  isLoading?: boolean;
+}
+
+// دکمه‌ی اصلی هر مرحله‌ی onboarding؛ چون داخل ناحیه‌ی overflow-y-auto صفحه است،
+// sticky bottom-0 نسبت به همون ناحیه چسبیده می‌مونه، نه اینکه ته لیست اسکرول بشه.
+export function StepFooterButton({
+  children,
+  isLoading,
+  disabled,
+  ...props
+}: StepFooterButtonProps) {
+  return (
+    <div className="sticky bottom-0 -mx-6 px-6 pt-4 pb-[calc(env(safe-area-inset-bottom)+16px)] bg-(--color-bg-primary) border-t border-(--color-border-subtle)">
+      <Button size="lg" isBlock isLoading={isLoading} disabled={disabled} {...props}>
+        {children}
+      </Button>
+    </div>
+  );
+}

src/components/onboarding/steps/BasicInfoStep.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState } from "react";
-import { Button } from "@/components/ui/Button";
+import { StepFooterButton } from "@/components/onboarding/StepFooterButton";
 import { TextField } from "@/components/ui/TextField";
 import { ErrorBanner } from "@/components/ui/ErrorBanner";
 import { cn } from "@/lib/utils";
@@ -79,9 +79,9 @@ export function BasicInfoStep({
 
       <ErrorBanner message={error} />
 
-      <Button size="lg" isBlock disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
+      <StepFooterButton disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
         بریم مرحله بعد
-      </Button>
+      </StepFooterButton>
     </div>
   );
 }

src/components/onboarding/steps/BodyInfoStep.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState } from "react";
-import { Button } from "@/components/ui/Button";
+import { StepFooterButton } from "@/components/onboarding/StepFooterButton";
 import { TextField } from "@/components/ui/TextField";
 import { ErrorBanner } from "@/components/ui/ErrorBanner";
 import { submitBodyInfo, FullProfile } from "@/lib/api";
@@ -58,9 +58,9 @@ export function BodyInfoStep({
         onChange={(e) => setWeight(e.target.value)}
       />
       <ErrorBanner message={error} />
-      <Button size="lg" isBlock disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
+      <StepFooterButton disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
         بریم مرحله بعد
-      </Button>
+      </StepFooterButton>
     </div>
   );
 }

src/components/onboarding/steps/EducationStep.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useEffect, useState } from "react";
-import { Button } from "@/components/ui/Button";
+import { StepFooterButton } from "@/components/onboarding/StepFooterButton";
 import { ErrorBanner } from "@/components/ui/ErrorBanner";
 import { FullScreenLoader } from "@/components/ui/FullScreenLoader";
 import { OptionPicker } from "@/components/ui/OptionPicker";
@@ -84,9 +84,9 @@ export function EducationStep({
 
       <ErrorBanner message={error} />
 
-      <Button size="lg" isBlock disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
+      <StepFooterButton disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
         بریم مرحله بعد
-      </Button>
+      </StepFooterButton>
     </div>
   );
 }

src/components/onboarding/steps/GenericOptionStep.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useEffect, useState } from "react";
-import { Button } from "@/components/ui/Button";
+import { StepFooterButton } from "@/components/onboarding/StepFooterButton";
 import { ErrorBanner } from "@/components/ui/ErrorBanner";
 import { FullScreenLoader } from "@/components/ui/FullScreenLoader";
 import { OptionPicker } from "@/components/ui/OptionPicker";
@@ -83,9 +83,9 @@ export function GenericOptionStep({
         max={max}
       />
       <ErrorBanner message={error} />
-      <Button size="lg" isBlock disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
+      <StepFooterButton disabled={!isValid} isLoading={loading} onClick={handleSubmit}>
         بریم مرحله بعد
-      </Button>
+      </StepFooterButton>
     </div>
   );
 }

src/components/onboarding/steps/PhotosStep.tsx

@@ -2,7 +2,7 @@
 
 import { useRef, useState } from "react";
 import { Plus, X } from "lucide-react";
-import { Button } from "@/components/ui/Button";
+import { StepFooterButton } from "@/components/onboarding/StepFooterButton";
 import { ErrorBanner } from "@/components/ui/ErrorBanner";
 import { confirmPhotosStep, uploadPhoto, deletePhoto, photoUrl, FullProfile, Photo } from "@/lib/api";
 import { formatApiError } from "@/lib/hooks";
@@ -102,9 +102,9 @@ export function PhotosStep({
 
       <ErrorBanner message={error} />
 
-      <Button size="lg" isBlock disabled={photos.length === 0} isLoading={loading} onClick={handleNext}>
+      <StepFooterButton disabled={photos.length === 0} isLoading={loading} onClick={handleNext}>
         بریم مرحله بعد
-      </Button>
+      </StepFooterButton>
     </div>
   );
 }

src/components/onboarding/steps/ProfileQuestionsStep.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useEffect, useState } from "react";
-import { Button } from "@/components/ui/Button";
+import { StepFooterButton } from "@/components/onboarding/StepFooterButton";
 import { ErrorBanner } from "@/components/ui/ErrorBanner";
 import { FullScreenLoader } from "@/components/ui/FullScreenLoader";
 import { cn } from "@/lib/utils";
@@ -139,9 +139,9 @@ export function ProfileQuestionsStep({
 
       <ErrorBanner message={error} />
 
-      <Button size="lg" isBlock disabled={!allAnswered} isLoading={loading} onClick={handleSubmit}>
+      <StepFooterButton disabled={!allAnswered} isLoading={loading} onClick={handleSubmit}>
         بریم مرحله بعد
-      </Button>
+      </StepFooterButton>
     </div>
   );
 }