refactor(electron): remove the configuration-preset system and read .env with dotenv

- delete electron/settings, the settings preload and window, the
  config-presets IPC channels, the Configuration menu and the preset
  watcher in scripts/electron-dev.mjs; config-manager.ts keeps only the
  user locale (still read from config-presets.json so the saved
  language survives)
- electron/main/env-loader.ts parses .env with dotenv instead of a
  hand-written parser; electron:compile bundles dependencies
- drop the unreachable development branches in port-manager.ts and
  next-server.ts, the duplicated locale normalisation in menu-i18n.ts,
  the unused electronAPI members, resources/entitlements.mac.plist
- scripts/copy-dereferenced.cjs shared by afterPack and
  prepare-electron-build
- CI: delete ci.yml, move type check and concurrency into test.yml,
  Node 24 everywhere, docker-build only on main
- remove unused dependencies (@xmldom/xmldom, remark-gfm,
  @anthropic-ai/tokenizer, @testing-library/user-event, concurrently,
  wait-on, eslint, eslint-config-next, @vitest/coverage-v8, shx),
  .eslintrc.json and the vitest coverage block

.github/workflows/docker-build.yml

@@ -4,14 +4,11 @@ on:
   push:
     branches:
       - main
-      - master
-      - dev
     tags:
       - 'v*'
   pull_request:
     branches:
       - main
-      - master
 
 env:
   REGISTRY: ghcr.io