Tests: Cross-browser Playwright matrix (Chromium / Firefox / WebKit)

[jjeff]

Problem

`playwright.config.ts` configures multiple projects but CI currently runs only on one browser per platform. Drag-and-drop has known browser-specific quirks (WebKit pointer events, Firefox dragend timing) that must be exercised pre-release.

Acceptance criteria

• CI runs Chromium, Firefox, and WebKit on every PR (sharded if necessary for runtime).
• Per-browser failures triaged: documented quirk + workaround, or filed bug.
• README badge showing browser support.