Update contributor docs for URL-first playback

docs/qa.md

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+# Bitvid Manual QA Script
+
+Run this checklist before publishing releases or merging changes that touch upload or playback flows. Copy/paste the steps into QA tickets when requesting validation.
+
+1. **Upload Modal Smoke Test**
+   - Open the Upload modal, verify title is required, and ensure either a hosted URL or a magnet (or both) must be supplied.
+   - Submit three variants: URL-only, magnet-only, and both URL + magnet. Confirm success messaging or event publication in each case.
+2. **URL-First Playback**
+   - For a post with both URL and magnet, play the video and confirm the hosted URL loads first (inspect network requests or logs).
+   - Simulate a dead URL (offline mode or request blocker) and confirm playback falls back to WebTorrent without breaking the UI.
+3. **Magnet Safety**
+   - Paste an encoded magnet and confirm `safeDecodeMagnet()` outputs the raw string before playback.
+   - Ensure `normalizeAndAugmentMagnet()` adds provided `ws=` / `xs=` hints while keeping the original `xt` hash intact.
+4. **P2P Hints & Trackers**
+   - Confirm magnets generated through the modal include HTTPS `ws=` seeds and optional HTTPS `xs=` torrent URLs when supplied.
+   - Check that tracker arrays originate from `js/constants.js` and only include WSS endpoints.
+5. **Cross-Browser Verification**
+   - Spot-check Chromium and Firefox (desktop) for console warnings about CORS, Range requests, or tracker connectivity.
+   - Optionally validate on mobile Safari/Chrome if the change targets mobile UX.
+
+Document findings (pass/fail notes plus relevant screenshots or logs) so they can be attached to release or PR notes.