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/.eslintrc.js

@@ -1,48 +0,0 @@
-module.exports = {
-  "root": true,
-  "parser": "@typescript-eslint/parser",
-  "parserOptions": {"project": ["./tsconfig.json"]},
-  "plugins": [
-    "@typescript-eslint"
-  ],
-  "ignorePatterns": ["node_modules/*", "build/*", "public/pb/*"],
-  "env": {
-    "jest": true
-  },
-  "rules": {
-    "array-bracket-spacing": ["error", "never"],
-    "arrow-spacing": ["error", {"before": true, "after": true}],
-    "block-spacing": ["error", "always"],
-    "brace-style": ["error", "1tbs", {"allowSingleLine": false}],
-    "comma-spacing": ["error", {"before": false, "after": true}],
-    "comma-style": ["error", "last"],
-    "computed-property-spacing": ["error", "never"],
-    "curly": ["error", "all"],
-    "dot-location": ["error", "property"],
-    "eol-last": ["error"],
-    "func-names": ["warn"],
-    "indent": ["error", 2, {"SwitchCase": 1}],
-    "key-spacing": ["error", {"beforeColon": false, "afterColon": true}],
-    "keyword-spacing": ["error"],
-    "linebreak-style": ["error", (process.platform === "win32" ? "windows" : "unix")],
-    "max-len": ["error", {"code": 140}],
-    "no-eq-null": ["off"],
-    "no-func-assign": ["error"],
-    "no-inline-comments": ["error"],
-    "no-mixed-spaces-and-tabs": ["error"],
-    "no-multi-spaces": ["error"],
-    "no-spaced-func": ["error"],
-    "no-trailing-spaces": ["error"],
-    "no-var": ["error"],
-    "object-curly-spacing": ["error", "always"],
-    "one-var": ["error", "never"],
-    "one-var-declaration-per-line": ["error"],
-    "quotes": ["error", "single"],
-    "semi-spacing": ["error", {"before": false, "after": true}],
-    "space-before-blocks": ["error"],
-    "space-before-function-paren": ["error", {"asyncArrow": "always", "anonymous": "never", "named": "never"}],
-    "space-in-parens": ["error", "never"],
-    "space-infix-ops": ["error"],
-    "space-unary-ops": ["error", {"words": true, "nonwords": false}]
-  }
-}