style: improve inline codespan contrast in markdown

[CES1919]

Motivation

• Inline markdown code (.codespan) was rendering with low-contrast/gray text in some themes, making important LLM output hard to read in the UI.

Description

• Update src/app.css to strengthen .codespan styling by setting a bold font weight and explicit light-mode foreground/background colors plus a subtle border, and add a .dark .codespan override for high contrast in dark mode; !important is used to ensure these values win over external markdown/theme CSS.

Testing

• Verified the change and file contents with nl -ba src/app.css | sed -n '326,360p' and confirmed the file was staged and committed with git add/git commit (commands executed successfully).

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Codex Task