fix(flex-fields): load third-party CSS by bundle name, the way abp-tree does

[duguankui]

Summary

• Select field types (SelectControlComponent, SelectSearchComponent) now lazy-load ng-zorro-antd-select.css by a fixed bundle name at init, through a new FlexFieldsStyleLoader — the same contract abp-tree in @abp/ng.components already uses for ng-zorro-antd-tree.css. Fixes the drift described in Angular: aggregate ng-zorro-antd style dependencies into one published stylesheet instead of per-consumer angular.json entries #232, where this repo's demo, vault-extract, and vault had each ended up with a different partial set of angular.json entries for ng-zorro-antd's CSS.
• An aggregated stylesheet shipped from the package (the fix originally proposed in Angular: aggregate ng-zorro-antd style dependencies into one published stylesheet instead of per-consumer angular.json entries #232) turned out not to work: ng-zorro-antd declares its per-component style exports with the less condition before style, so a bare @import of that CSS cannot resolve under the Angular CLI's stylesheet bundler. See the issue comments for the full analysis.
• @dignite/ng.flex-fields-ckeditor moves onto the same loader. It used to @import ckeditor5's 241 KB UI stylesheet into a component style, which ng-packagr compiled straight into the published bundle (471 KB, shipped to every host regardless of whether a rich-text field was ever used) and pinned to whatever ckeditor5 version the package was built against, independent of the host's own installed copy the editor's JavaScript already comes from at runtime. It now asks for the host's own ckeditor5.css by bundle name instead.
• A host that forgets the angular.json entry now gets one console error naming the missing file and quoting the exact entry to add, instead of a silently unstyled control (Select) or a blank/collapsed editor (CKEditor). DISABLE_FLEX_FIELDS_STYLE_LOADING_TOKEN opts a host out when it bundles the CSS another way.
• Dead ng-zorro-antd-tree-select entry removed from the demo's angular.json — nothing in the workspace renders nz-tree-select.
• READMEs for all three flex-fields Angular packages document the angular.json entries a host needs, under a new ### Styles section.

Closes #232.

Test plan

• yarn test (flex-fields): 43 files / 302 tests pass
• ng test flex-fields-ckeditor: 6 files / 39 tests pass
• ng test flex-fields-file-explorer: 4 files / 21 tests pass
• yarn lint: all four projects pass
• yarn build:lib / build:lib:file-explorer / build:lib:ckeditor, then check-angular-package-deps.mjs on all three dist folders: pass
• yarn build / yarn build:prod: pass; ng-zorro-antd-select.css, ng-zorro-antd-tree.css, ckeditor5.css all emitted under their literal names, none referenced from index.html
• ckeditor-ckeditor fesm bundle: 471 KB → 36 KB; demo initial bundle: 2.20 MB → 1.98 MB (back under the 2 MB budget)
• Manual smoke test of a real CKEditor field (Basic and Full toolbar, light and dark theme) against a running .NET host — not exercised in this PR, worth doing before merge