[UX] Add subtle background color for response area #110

Problem

Response text area has no visual distinction from tool output.

Solution

Add subtle background color to response/prose text using the theme system.

Implementation

• Use color from packages/opencode/src/cli/lite/theme.ts
• Apply to prose/response text blocks in liveblock.ts or index.ts
• Should be subtle, not distracting
• Must use theme system to prepare for full theming support (like opencode TUI)

Acceptance Criteria

• Response text has visible background color
• Color comes from theme.ts (not hardcoded ANSI codes)
• Theme system is extensible for future theming parity with TUI
• Works in both light and dark terminals