fix: user message text invisible in multimodal content

[ulivz]

Problem

User messages with both text and images show invisible text due to color contrast issues.

Root Cause

1. User messages use dark background (#141414) with white text
2. Text-only: renders as raw text, inherits white color ✅
3. Text+image: uses MarkdownRenderer without forceDarkTheme, renders gray text on dark background ❌

Technical Details

MultimodalContent.tsx calls MarkdownRenderer without theme:

<MarkdownRenderer content={part.text} /> // Missing forceDarkTheme

MarkdownRenderer defaults to light theme:

const themeClass = forceDarkTheme ? 'dark' : 'light';
// Results in text-gray-800 (dark gray) on #141414 background

Solution

Pass forceDarkTheme={true} to MarkdownRenderer in MultimodalContent.tsx:

<MarkdownRenderer content={part.text} forceDarkTheme={true} />