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>

tests/editor_sources.py

@@ -2,8 +2,10 @@
 
 The page is a Vue app in web/ (app 0.2.73); `npm run build` writes it to screen_manager/app/static. Tests look at
 the source, not the build: every .ts and .vue file under web/src as one text (SCRIPT, which is also the markup:
-a .vue file carries its template), and the stylesheets as another (CSS).
+a .vue file carries its template), and the stylesheets as another (CSS). Its English words are in the `editor` section
+of screen_manager/translations/en.json since app 0.2.90: text() reads one of them.
 """
+import json
 from pathlib import Path
 
 ROOT = Path(__file__).resolve().parents[1]
@@ -23,3 +25,14 @@ def source(name):
 
 def component(name):
     return source(f'components/{name}.vue')
+
+
+TEXTS = json.loads((ROOT / 'screen_manager/translations/en.json').read_text(encoding='utf-8'))['editor']
+
+
+def text(key):
+    """One English text of the editor by its key under `editor`, e.g. 'empty.choose.title'."""
+    node = TEXTS
+    for part in key.split('.'):
+        node = node[int(part)] if isinstance(node, list) else node[part]
+    return node