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.

.github/dependabot.yml

@@ -39,13 +39,3 @@ updates:
       interval: "weekly"
     # Limit the amout of open PR's (default = 5, disabled = 0, security updates are not impacted)
     open-pull-requests-limit: 2
-
- # # Enable version updates for npm
- # - package-ecosystem: "npm"
- #   # Look for `package.json` and `lock` files in the `webclient` subdirectory
- #   directory: "/webclient"
- #   # Check the npm registry for updates once a week
- #   schedule:
- #     interval: "weekly"
- #   # Limit the amout of open PR's (default = 5, disabled = 0, security updates are not impacted)
- #   open-pull-requests-limit: 5