Improve dark-mode code readability and build reliability

[vitek-karas]

Why

• Dark-mode markdown code snippets were hard to read due to low contrast and an error-like red appearance.
• Mac Catalyst publish surfaced noisy metadata warnings for the SDK's intermediate Copilot CLI binary.
• Test builds were susceptible to CLI download timeouts because the SDK timeout value is interpreted in milliseconds.

What changed

1) Chat markdown code styling (PolyPilot/Components/ChatMessageList.razor.css)

• Set inline and fenced code text to var(--text-bright) for stronger contrast on dark surfaces.
• Added subtle borders to code surfaces for better visual separation.
• Ensured pre code inherits fenced block colors and avoids duplicate styling.

2) Mac Catalyst publish metadata cleanup (PolyPilot/PolyPilot.csproj)

• Excluded the intermediate SDK Copilot CLI artifact from publish item lists on Mac Catalyst.
• Preserved explicit CLI copy behavior while preventing duplicate/noisy publish metadata entries.

3) Reliable CLI download timeout (Directory.Build.props)

• Added CopilotCliDownloadTimeout=600000 so the SDK DownloadFile task gets a real 10-minute timeout.
• This avoids the previous effective ~600ms timeout that caused flaky restore/build failures.

Validation

• dotnet test --nologo in PolyPilot.Tests passed: 713 passed, 0 failed.
• Relaunched PolyPilot and visually verified assistant markdown code blocks are readable in dark mode.