[Toolkit][Shadcn] Add code-block recipe - #4100

| Q              | A
| -------------- | ---
| Bug fix?       | no
| New feature?   | yes
| Deprecations?  | no
| Documentation? | yes
| Issues         | Part of symfony#3233
| License        | MIT

The Shadcn kit had no `code-block` recipe, the component Shadcn uses to display code, terminal output, logs and diffs.

This adds it, ported from the Nova style: `CodeBlock` (with a `destructive` variant and an `sm` size), `CodeBlock:Header`, `CodeBlock:Title`, `CodeBlock:Actions`, `CodeBlock:CopyButton`, `CodeBlock:Content`, `CodeBlock:Line` (`added`, `removed` and `highlighted` variants) and `CodeBlock:Footer`. `CodeBlock:Content` takes a `code` string or a list of `CodeBlock:Line`, and supports `wrap`, `lineNumbers` and `startLine`. Line numbers and diff markers are generated content, left out of a copied selection, and blank lines survive the copy. The content is always `dir="ltr"`, and the header follows the page direction.

A `code-block` Stimulus controller replaces the React state of upstream: `copy` (check icon, "Copied" label and live region for 2 seconds), `download`, `toggleWrap` (syncs `aria-pressed`) and `toggleExpanded` (toggles `data-expanded` on the content the button controls, for Show More). The edges fade with the `scroll-fade-*` utilities of the `shadcn` package the kit already depends on.

Choices that differ from upstream:

- The copy text is read from a `data-code-block-text` attribute rather than a Stimulus param, since Stimulus parses params as JSON and would turn a JSON snippet into an object.
- `scrollbar-thin` only exists since Tailwind 4.3. The Toolkit builds with Tailwind 4.2. The recipe uses `[scrollbar-width:thin]` instead, which works in both versions.
- Syntax highlighting is not ported: upstream does it with Shiki in a React hook. The README has a Syntax Highlighting section instead, which shows how to pass lines highlighted by the application to `CodeBlock:Line`. It lists Phiki, tempest/highlight and Shiki as options. The Languages, Terminal Output, Streaming and Markdown sections are left out. Examples that only used highlighting for looks show plain text.
- The Typeset example is left out: it relies on the `typeset` stylesheet of the Shadcn docs site, shipped neither by the kit nor by the `shadcn` package.

The README carries the demo and every other example of the upstream documentation: Basic, Header, Actions, Floating Actions, Line Numbers, Start Line, Wrap, Highlighted Lines, Diff, Height, Show More, Max Height, Size, Destructive, Trace, Tabs, Message and RTL, plus an Accessibility section. A `lucide:wrap-text` icon fixture is added.

The `trace` README used `<pre>` elements styled like a code block, since the kit had none. They are replaced with `CodeBlock`, as upstream does.

Checks run:

- `ComponentsRenderingTest`: snapshots of the 19 examples, and the full Toolkit PHPUnit suite (1408 tests) passes.
- Playwright: 50 tests for the recipe (screenshots in light and dark mode, plus specs for copying, the reset after 2 seconds, a selection without line numbers, the wrap toggle, the download, Show More, the higher cap and the floating actions on focus), and the 40 tests of `trace` pass.
- `ux-toolkit-kit-lint --fail-on-warning`, `oxfmt` and `oxlint` are clean.