Node 22+ everywhere, LF text on Windows checkouts, CI actions v7

- Node minimum is 22 in package.json engines, the start scripts, the service installer, setup and doctor,
  matching the README, the release notes and the CI matrix (22 / 24).
- .gitattributes: CSS, HTML, JSON, Markdown and YAML check out with LF, so `node --test` on Windows matches the
  CSS rules the UI tests read; the Windows test step is no longer continue-on-error.
- CI: actions/checkout@v7 and actions/setup-node@v7 (Node 24 runtime).

docs/DESIGN.md

@@ -27,7 +27,7 @@ Out of scope v1: matchmaking queue, training/tutorial, DIY (甄选) slots (the 4
 
 ## 1. Tech stack
 
-- **Node.js ≥ 18** (tested on 22), ESM (`"type": "module"`), single dependency `ws@8`. No bundler, no TypeScript. JSDoc types where helpful.
+- **Node.js ≥ 22** (CI: 22 and 24), ESM (`"type": "module"`), single dependency `ws@8`. No bundler, no TypeScript. JSDoc types where helpful.
 - **Server-authoritative simulation.** Clients send *intents*; the server validates, mutates state and pushes state/snapshots.
 - **Client:** static files, native ES modules. Vendored libs in `public/vendor/`: `pixi.min.js` (PixiJS **7.4.2** UMD, global `PIXI`), `pixi-spine.js` (**4.0.6** UMD, `PIXI.spine`), `preact.module.js` + `hooks.module.js` + `htm.module.js` (Preact 10 + htm, no build step). No CDN at runtime (LAN play must work offline).
 - **Shared code** in `shared/` is imported by both server and browser (pure ESM, no Node APIs).