P2 frontend: enable standalone output for smaller image

## Phase / Priority / Area
Phase 15 · Priority P2 · Area frontend

## Scope
`frontend/next.config.mjs`, `frontend/Dockerfile`, `frontend/src/components/VideoPlayer.tsx:34`

## Goal
FE image <200MB.

## Acceptance Criteria
- [ ] `output: standalone`, `docker images frontend` <200MB, `maxBufferLength:10` сохранен

## Проверки
`docker build && docker images`, `make e2e` HLS
