Speak the editor in each person's Home Assistant language: every text in the editor section of en.json, the mockup in the screens' language, and one Language & region card for all screens

- vue-i18n 11 with the Composition API: English comes with the page, any other language loads the first time it is
  needed from screen_manager/translations, and a text a language lacks shows in English. The build keeps only _meta
  and editor of each file (vite.config.ts), never the screens' or the add-on's texts.
- The editor follows the language of the user's Home Assistant profile (<html lang> of Home Assistant's page), else
  the browser's; plural forms follow the file's _meta.plural, with the screens' rules.
- Every request tells the add-on the editor's language (X-ESP-Screens-Language), so its own words can follow it.
- The mockup draws the screens' words (Unavailable, Page 2 ›, the date, "5 min ago") in the screens' language, with
  their clock and number format.
- Settings → Language & region: the screen language, time format and number format for every screen, with the user's
  own profile offered when it differs. The per-screen clock choice goes; the top bar points there. A screen that
  still needs the new language says so in its update badge.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/vitest.config.ts

@@ -1,13 +1,16 @@
 import vue from "@vitejs/plugin-vue";
 import { defineConfig } from "vitest/config";
+import { editorTexts } from "./vite.config";
 
 // Unit and component tests of the editor: the grid rules, the top bar rules, the store and the components,
-// in jsdom. `npm test` runs them; `npm run test:watch` while developing.
+// in jsdom. `npm test` runs them; `npm run test:watch` while developing. The components mount with the
+// editor's texts (tests/setup.ts), in English.
 export default defineConfig({
-  plugins: [vue()],
+  plugins: [editorTexts(), vue()],
   test: {
     environment: "jsdom",
     include: ["tests/**/*.spec.ts"],
+    setupFiles: ["tests/setup.ts"],
     restoreMocks: true,
   },
 });