D-3: DesignSystem — add BlockCodeContainer primitive

[kirich1409]

Description

Add BlockCodeContainer — presentational container for code-block rendering (monospaced font, dark surface, rounded corners, horizontal scroll for long lines). Used by DialogueMarkdownView for fenced code blocks, by BashWidget for command/output display, and by specialized tool widgets rendering code.

Spec: Epic #250 §6 (UI concept — specialized tool widgets).

Scope

Create MacApp/Packages/DesignSystem/Sources/DesignSystem/Components/BlockCodeContainer.swift:

```swift
public struct BlockCodeContainer<Content: View>: View {
public init(
language: String? = nil, // displayed in top-right corner (e.g., "swift")
showLineNumbers: Bool = false,
@ViewBuilder content: () -> Content
)
}
```

• Background: DS.Color.codeBackground (semantic token — new if missing; adapts light/dark/HCR).
• Radius: DS.Radius.md (.continuous).
• Padding: DS.Spacing.sm internal.
• Language badge: top-right corner, DS.Text.caption2 monospaced, tinted DS.Color.textTertiary.
• showLineNumbers — prefixes each line with 1-indexed counter in DS.Color.textTertiary.
• .textSelection(.enabled) on content so user can copy code.
• Horizontal scroll for lines longer than container width (ScrollView(.horizontal, showsIndicators: false)).
• No syntax highlighting in MVP (monochrome). Highlight plugs into content via consumer — the container is presentational only.

Acceptance Criteria

• BlockCodeContainer.swift with public init(language:showLineNumbers:content:).
• Renders correctly with 1, 10, 100, 1000 lines of code; horizontal scroll works for lines > 200 chars.
• Language badge displays when language != nil, hidden otherwise.
• Line numbers column is fixed-width, aligns with content.
• .textSelection(.enabled) on content — verified via VoiceOver + ⌘C copies selection.
• DS.Color.codeBackground token added to DS.Color if missing (4 appearances in Asset Catalog).
• Added to DesignSystemCatalog — previews with / without language, with / without line numbers, light/dark/HCR.
• Owner approval for ds-api.

Relationships

• Parent issue: EPIC: Dialogue — structured chat UI for agent sessions #250
• Blocks: D-13 (DialogueMarkdownView), D-16 (Specialized tool widgets — Bash, Read)