Add Vitest tests for the languages, router, footer and pages, and a lint:check script

tests/setup.js

@@ -0,0 +1,88 @@
+// Runs before every test file (see vitest.config.js).
+
+// jsdom has no matchMedia, which the theme toggle reads.
+if (!window.matchMedia) {
+  window.matchMedia = (query) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => false,
+  })
+}
+
+// jsdom has no IntersectionObserver / ResizeObserver, which some views use.
+for (const name of ['IntersectionObserver', 'ResizeObserver']) {
+  if (!window[name]) {
+    window[name] = class {
+      observe() {}
+      unobserve() {}
+      disconnect() {}
+      takeRecords() {
+        return []
+      }
+    }
+  }
+}
+
+// jsdom cannot draw: canvas.getContext() is null, and the statistics charts call
+// ctx.scale() straight away. A stand-in whose every method does nothing lets
+// the page run; what it would have drawn is not what is tested.
+HTMLCanvasElement.prototype.getContext = function getContext() {
+  const noop = () => {}
+  return new Proxy(
+    { canvas: this },
+    {
+      get: (target, prop) =>
+        prop in target ? target[prop] : prop === 'measureText' ? () => ({ width: 0 }) : noop,
+      set: (target, prop, value) => {
+        target[prop] = value
+        return true
+      },
+    }
+  )
+}
+
+// Node 22+ has its own `localStorage` global that wins over jsdom's and does
+// nothing useful unless node is started with --localstorage-file. A plain
+// in-memory one means the tests do not depend on the node version or a flag.
+class MemoryStorage {
+  #map = new Map()
+  getItem(key) {
+    return this.#map.has(key) ? this.#map.get(key) : null
+  }
+  setItem(key, value) {
+    this.#map.set(key, String(value))
+  }
+  removeItem(key) {
+    this.#map.delete(key)
+  }
+  clear() {
+    this.#map.clear()
+  }
+  key(index) {
+    return Array.from(this.#map.keys())[index] ?? null
+  }
+  get length() {
+    return this.#map.size
+  }
+}
+
+const memoryStorage = new MemoryStorage()
+Object.defineProperty(globalThis, 'localStorage', {
+  value: memoryStorage,
+  configurable: true,
+  writable: true,
+})
+Object.defineProperty(window, 'localStorage', {
+  value: memoryStorage,
+  configurable: true,
+  writable: true,
+})
+
+beforeEach(() => {
+  localStorage.clear()
+})