Add thinkingText theme token, fix streaming toggle bug

- Add configurable thinkingText color for thinking blocks (defaults to muted)
- Make 'Thinking...' label italic when collapsed
- Fix Ctrl+T during streaming hiding the current message
- Track streamingMessage to properly re-render on toggle

Based on #366 by @paulbettner

packages/coding-agent/src/modes/interactive/theme/dark.json

@@ -29,6 +29,7 @@
 		"muted": "gray",
 		"dim": "dimGray",
 		"text": "",
+		"thinkingText": "gray",
 
 		"selectedBg": "selectedBg",
 		"userMessageBg": "userMsgBg",