Remove webclient (extracted to Webatrice repo) (#6848)

The webclient has been extracted to https://github.com/seavor/Webatrice
and the Playwright e2e suite has moved to Sockatrice. Cockatrice keeps
no copy.

Deleted:
- webclient/ (entire tree, 349 files)
- .github/workflows/web-build.yml, web-lint.yml
- .husky/pre-commit (the only tracked husky hook; managed entirely
  from webclient's package.json which no longer exists)

Edited:
- .tx/config: dropped the webclient resource block; Webatrice/.tx/config
  now manages its own translations
- .github/workflows/translations-pull.yml: removed webclient locales
  from add-paths
- .github/workflows/desktop-build.yml, desktop-lint.yml: removed dead
  '!webclient/**' and '!.husky/**' path-filter exclusions
- .github/dependabot.yml: removed commented-out npm/webclient block
- Doxyfile: removed webclient/ from EXCLUDE list
- .ci/release_template.md: dropped Webatrice section (Webatrice now
  cuts its own releases)
- README.md: dropped 'first work on a webclient' line, added Webatrice
  to Related Repositories, updated translation paragraph and build
  badges

History preserved: every webclient commit remains recoverable via
git log on master before this commit.

webclient/public/manifest.json

@@ -1,25 +0,0 @@
-{
-  "short_name": "React App",
-  "name": "Create React App Sample",
-  "icons": [
-    {
-      "src": "favicon.ico",
-      "sizes": "64x64 32x32 24x24 16x16",
-      "type": "image/x-icon"
-    },
-    {
-      "src": "logo192.png",
-      "type": "image/png",
-      "sizes": "192x192"
-    },
-    {
-      "src": "logo512.png",
-      "type": "image/png",
-      "sizes": "512x512"
-    }
-  ],
-  "start_url": ".",
-  "display": "standalone",
-  "theme_color": "#000000",
-  "background_color": "#ffffff"
-}