# 4K support in video downloads

## Purpose

Add 4K (2160p) and 2K (1440p) as selectable download quality tiers, and update marketing
copy that currently caps the advertised resolution at 1080p.

## Context

The download pipeline already resolves quality generically:

- `src/lib/ytdlp-options.ts` exports `QUALITIES`, a fixed list consumed by the probe
  (`src/lib/ytdlp-probe.ts`) to compute which qualities are actually available for a given
  source video (filtered against the max height yt-dlp reports), and by the UI
  (`src/components/SubmitForm.tsx`, via `capabilities.availableQualities`) to render the
  quality `<select>`.
- `src/lib/ytdlp.ts` (`buildYtdlpArgs`) turns the selected quality string into a yt-dlp
  format selector via `height<=?${quality.replace('p', '')}`, with no hardcoded ceiling.

Because both the filtering and the format-selector logic are already generic over the
quality string, no logic changes are required — only the `QUALITIES` list itself, its
tests, and marketing copy need to change.

## Changes

### 1. Quality list — `src/lib/ytdlp-options.ts`

```ts
export const QUALITIES = ['best', '2160p', '1440p', '1080p', '720p', '480p', '360p'] as const
```

### 2. Tests

- `src/lib/__tests__/ytdlp-probe.test.ts`: add a case with a 4K-height fixture
  (`height: 2160`) asserting `availableQualities` includes `'2160p'` and `'1440p'`. The
  existing 1080p-source case is unaffected (2160p/1440p correctly excluded since
  `maxHeight` there is 1080).
- `src/lib/__tests__/ytdlp.test.ts`: add a case for `quality: '2160p'` asserting the built
  args contain `height<=?2160`, mirroring the existing `1080p` case.

### 3. Marketing copy — `messages/{en,fr,es,it}.json`

Replace the "up to 1080p" claim (and localized equivalents) with "up to 4K" in the 5
affected keys, in each of the 4 locale files: `formatsDesc`, `step2Desc`, `a4`, and the two
`faqA2` occurrences (home FAQ + platform-page FAQ). Only the resolution figure changes;
surrounding wording stays identical.

## Out of scope

- No change to the download pipeline, worker, or database schema — the format selector and
  probe filtering are already resolution-agnostic.
- No change to per-platform SEO page copy (`src/lib/downloader-platforms.ts`) — it does not
  mention a specific resolution ceiling.