feat(e2e): Playwright + pixelmatch stack for pixel-perfect visual regression
- playwright.config.visual.ts: dedicated config, viewport 1280x720, Chromium only,
snapshots in e2e/tests/visual/__snapshots__
- e2e/tests/visual/visual-regression.spec.ts: login, register, dashboard (full/header/sidebar),
player bar, playlists, 404, mobile/tablet viewports; dark theme + reduceMotion
- scripts/visual-diff.js: optional pixelmatch script to generate diff image from two PNGs
- docs/VISUAL_TESTING_STRATEGY.md: strategy, commands, CI, workflow
- npm scripts: test:visual, test:visual:update, test:visual:report
- deps: pixelmatch, pngjs; @playwright/test aligned to 1.58.1
- baseline snapshots added for login, dashboard, playlists, 404, viewports
Co-authored-by: Cursor <cursoragent@cursor.com>
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