build(test): run unit tests with vitest and jsdom

Karma needed headless Chrome. Vitest with jsdom runs with no browser.
Lighthouse CI now gets Chrome from @puppeteer/browsers.
Two CSS font tests are dropped: jsdom does not apply real CSS.

Dockerfile

@@ -1,8 +1,6 @@
 # Build
 FROM node:22-alpine AS build
 WORKDIR /app
-# The production build never runs tests, so skip Puppeteer's Chrome download.
-ENV PUPPETEER_SKIP_DOWNLOAD=1
 COPY package*.json ./
 RUN npm ci
 COPY . .