feat(interview): 라이브 스테이지 웹캠 셀프뷰(PiP) 추가

frontend/src/features/interview/lib/media/useWebcamPreview.ts

@@ -0,0 +1,52 @@
+import { useCallback, useEffect, useRef, useState } from 'react'
+
+export type WebcamState = 'idle' | 'requesting' | 'live' | 'denied' | 'unsupported'
+
+// 라이브 면접 중 본인 카메라 미리보기(PiP) 전용 훅.
+// 실제 스트림 전송·분석은 범위 밖 — 로컬 미리보기만 담당한다.
+export function useWebcamPreview() {
+  const videoRef = useRef<HTMLVideoElement | null>(null)
+  const streamRef = useRef<MediaStream | null>(null)
+  const [state, setState] = useState<WebcamState>('idle')
+
+  const stop = useCallback(() => {
+    streamRef.current?.getTracks().forEach((t) => t.stop())
+    streamRef.current = null
+    if (videoRef.current) {
+      videoRef.current.srcObject = null
+    }
+    setState('idle')
+  }, [])
+
+  const start = useCallback(async () => {
+    if (!navigator.mediaDevices?.getUserMedia) {
+      setState('unsupported')
+      return
+    }
+    setState('requesting')
+    try {
+      const stream = await navigator.mediaDevices.getUserMedia({ video: true })
+      streamRef.current = stream
+      setState('live')
+    } catch {
+      setState('denied')
+    }
+  }, [])
+
+  // 스트림 준비 후 비디오 엘리먼트가 마운트되는 경우까지 커버해 srcObject 바인딩.
+  useEffect(() => {
+    if (state === 'live' && videoRef.current && streamRef.current) {
+      videoRef.current.srcObject = streamRef.current
+    }
+  }, [state])
+
+  // 언마운트 시 트랙 정리 (카메라 LED 가 계속 켜지는 것 방지 — 필수).
+  useEffect(
+    () => () => {
+      streamRef.current?.getTracks().forEach((t) => t.stop())
+    },
+    [],
+  )
+
+  return { videoRef, state, start, stop }
+}

frontend/src/features/interview/ui/live/InterviewStage.tsx

@@ -8,6 +8,7 @@ import { ConnectionBanner } from './ConnectionBanner'
 import { AnswerComposer } from './AnswerComposer'
 import { StageQuestion } from './StageQuestion'
 import { InterviewerAvatar } from './InterviewerAvatar'
+import { WebcamSelfView } from './WebcamSelfView'
 import { TranscriptDrawer } from './TranscriptDrawer'
 
 const BG = '/interview-session-background.png'
@@ -118,6 +119,10 @@ export function InterviewStage({
         )}
       </div>
 
+      <div className="absolute bottom-24 right-4 z-20 sm:bottom-28">
+        <WebcamSelfView />
+      </div>
+
       <div className="relative z-10">
         <AnswerComposer
           disabled={awaitingQuestion || connection !== 'open'}

frontend/src/features/interview/ui/live/WebcamSelfView.test.tsx

@@ -0,0 +1,65 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { WebcamSelfView } from './WebcamSelfView'
+
+function mockMediaDevices(getUserMedia: ReturnType<typeof vi.fn>) {
+  Object.defineProperty(navigator, 'mediaDevices', {
+    value: { getUserMedia },
+    configurable: true,
+  })
+}
+
+describe('WebcamSelfView', () => {
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('켜기 클릭 시 스트림을 받아 live 가 되고, 끄기 클릭 시 트랙을 정지한다', async () => {
+    const stop = vi.fn()
+    const stream = { getTracks: () => [{ stop }] }
+    mockMediaDevices(vi.fn().mockResolvedValue(stream))
+
+    render(<WebcamSelfView />)
+    await userEvent.click(screen.getByRole('button', { name: '카메라 켜기' }))
+
+    const off = await screen.findByRole('button', { name: '카메라 끄기' })
+    await userEvent.click(off)
+    expect(stop).toHaveBeenCalled()
+  })
+
+  it('권한 거부 시 안내를 보여준다', async () => {
+    mockMediaDevices(vi.fn().mockRejectedValue(new Error('denied')))
+
+    render(<WebcamSelfView />)
+    await userEvent.click(screen.getByRole('button', { name: '카메라 켜기' }))
+
+    expect(await screen.findByText('권한 거부됨')).toBeInTheDocument()
+  })
+
+  it('언마운트 시 트랙을 정지한다', async () => {
+    const stop = vi.fn()
+    const stream = { getTracks: () => [{ stop }] }
+    mockMediaDevices(vi.fn().mockResolvedValue(stream))
+
+    const { unmount } = render(<WebcamSelfView />)
+    await userEvent.click(screen.getByRole('button', { name: '카메라 켜기' }))
+    await screen.findByRole('button', { name: '카메라 끄기' })
+
+    unmount()
+    expect(stop).toHaveBeenCalled()
+  })
+})
+
+beforeEach(() => {
+  // jsdom 은 HTMLVideoElement.srcObject 를 구현하지 않으므로 setter 를 무해하게 stub.
+  if (!Object.getOwnPropertyDescriptor(HTMLMediaElement.prototype, 'srcObject')) {
+    Object.defineProperty(HTMLMediaElement.prototype, 'srcObject', {
+      configurable: true,
+      get() {
+        return null
+      },
+      set() {},
+    })
+  }
+})

frontend/src/features/interview/ui/live/WebcamSelfView.tsx

@@ -0,0 +1,51 @@
+import { useWebcamPreview } from '../../lib/media/useWebcamPreview'
+
+function CameraOffIcon() {
+  return (
+    <svg width="22" height="22" viewBox="0 0 24 24" fill="currentColor" aria-hidden>
+      <path d="M2.1 3.51 1 4.62l4.51 4.51A1 1 0 0 0 5 10v8a2 2 0 0 0 2 2h10c.21 0 .41-.03.6-.09l1.78 1.78 1.11-1.11L2.1 3.51ZM17 8.5V6a2 2 0 0 0-2-2H8.83l9.34 9.34A1 1 0 0 0 19 13l3 2V7l-3 2a1 1 0 0 0-2-.5Z" />
+    </svg>
+  )
+}
+
+const PLACEHOLDER: Record<string, string> = {
+  idle: '카메라 꺼짐',
+  requesting: '연결 중…',
+  denied: '권한 거부됨',
+  unsupported: '미지원 기기',
+}
+
+// 면접 스테이지에 떠 있는 본인 카메라 미리보기 카드. 위치는 호출부(InterviewStage)가 결정.
+export function WebcamSelfView() {
+  const { videoRef, state, start, stop } = useWebcamPreview()
+  const live = state === 'live'
+
+  return (
+    <div className="w-36 overflow-hidden rounded-xl border border-white/50 bg-sage-900/80 shadow-lg backdrop-blur-md sm:w-44">
+      <div className="relative aspect-video">
+        <video
+          ref={videoRef}
+          autoPlay
+          muted
+          playsInline
+          aria-label="내 카메라 미리보기"
+          className={['h-full w-full -scale-x-100 object-cover', live ? '' : 'hidden'].join(' ')}
+        />
+        {!live && (
+          <div className="flex h-full w-full flex-col items-center justify-center gap-1 text-white/70">
+            <CameraOffIcon />
+            <span className="text-[11px]">{PLACEHOLDER[state]}</span>
+          </div>
+        )}
+      </div>
+      <button
+        type="button"
+        onClick={live ? stop : start}
+        disabled={state === 'requesting' || state === 'unsupported'}
+        className="w-full bg-black/30 py-1 text-[11px] font-medium text-white transition-colors hover:bg-black/40 disabled:opacity-60"
+      >
+        {live ? '카메라 끄기' : '카메라 켜기'}
+      </button>
+    </div>
+  )
+}