Improve markdown styling and enable text selection on mobile

- Fix agent chat bottom scroll padding by adding overflow hidden to inputAreaContainer
- Create shared markdown-styles.ts with comprehensive styling for tables, headings, code blocks, blockquotes, and horizontal rules
- Enable text selection in AssistantMessage and AgentThoughtMessage by overriding markdown render rules with selectable={true}
- Path chips remain non-selectable to preserve onPress navigation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/app/src/components/agent-input-area.tsx

@@ -1139,6 +1139,7 @@ const styles = StyleSheet.create(((theme: any) => ({
     minHeight: FOOTER_HEIGHT,
     alignItems: "center",
     width: "100%",
+    overflow: "hidden",
   },
   inputAreaContent: {
     width: "100%",