docs: add a JDownloader quick reference

Covers why fetching goes through JDownloader rather than Instaloader (the
instagram.com vs CDN split, and what the metadata gap actually costs), the
settings that matter, cookie handling, the two-URL workflow, jd2-sync usage,
the expected on-disk layout, and what to do when something breaks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011uBWhwV3wFQ5MBCcMHHem7

CLAUDE.md

@@ -16,7 +16,8 @@ InstaArchive Viewer is a React 19 + Vite 6 PWA for browsing archived Instagram d
 - `npm test` / `npm run test:watch` — vitest
 - `npx vitest run src/lib/archive-patterns.test.ts` — a single test file
 - `npm run jd2 -- --archives <dir> --dry-run` — generate JDownloader `.crawljob`
-  files for every profile on disk (see `scripts/jd2-sync.ts`)
+  files for every profile on disk (see `scripts/jd2-sync.ts` and
+  `docs/jdownloader.md`)
 
 Local development usually needs both `npm run dev` and `npm run server`. Local-folder mode works without the backend; server-mode archives do not.