Scope dark pre styling to .code-block opt-in class

[Copilot]

The global pre { ... } rule in app.css forced dark background/light text onto every <pre> element, overriding Bootstrap utilities like bg-light (Agent15 YAML block) and creating near-invisible text when combined with .response-text's dark color.

Changes

• app.css: Renamed pre { ... } → .code-block { ... } and pre code { ... } → .code-block code { ... }, making dark code block styling opt-in rather than global

Dark blocks now require an explicit class:

<!-- opt in to dark styling -->
<pre class="code-block"><code>...</code></pre>

<!-- light blocks (e.g. Agent15 YAML) now render correctly without override -->
<pre class="bg-light border rounded p-2 small"><code>...</code></pre>

Existing usages (.thread-log, .response-text, Bootstrap classes) all carry their own styling and are unaffected.

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