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/workflows/web-build.yml

@@ -1,54 +0,0 @@
-name: Build Web
-
-on:
-  push:
-    branches:
-      - master
-    paths:
-      - '.husky/**'
-      - 'webclient/**'
-      - '!**.md'
-      - '.github/workflows/web-build.yml'
-  pull_request:
-    paths:
-      - '.husky/**'
-      - 'webclient/**'
-      - '!**.md'
-      - '.github/workflows/web-build.yml'
-
-jobs:
-  build-web:
-    name: React (Node ${{matrix.node_version}})
-
-    runs-on: ubuntu-latest
-
-    defaults:
-      run:
-        working-directory: webclient
-
-    strategy:
-      fail-fast: false
-      matrix:
-        node_version:
-          - 16
-          - lts/*
-
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v6
-
-      - name: Setup Node.js
-        uses: actions/setup-node@v6
-        with:
-          node-version: ${{matrix.node_version}}
-          cache: 'npm'
-          cache-dependency-path: 'webclient/package-lock.json'
-
-      - name: Install dependencies
-        run: npm clean-install
-
-      - name: Build app
-        run: npm run build
-
-      - name: Test app
-        run: npm run test