fix: tab isolation + change log in chat not preview

Tab Isolation:
- Canvas visibility now uses activeTab.showCanvas directly (not local state)
- Chat panel width also uses tab state for proper isolation
- New tabs explicitly have showCanvas: false, previewData: null

Change Log Fix:
- Updated all AI prompts to emphasize change log OUTSIDE [/PREVIEW]
- Added example format showing change log after closing tag
- Change log now appears in chat stream, not rendered in canvas

lib/services/zai-plan.ts

@@ -866,8 +866,16 @@ STRICT OUTPUT FORMAT:
 ACTUAL_FUNCTIONAL_CODE_OR_DATA
 [/PREVIEW]
 
-CHANGE LOG:
-After the preview block, provide a brief bulleted "Change Log" explaining exactly what elements were modified, added, or removed.
+CHANGE LOG (CRITICAL - MUST BE OUTSIDE PREVIEW):
+- After the [/PREVIEW] closing tag, provide a brief bulleted "Change Log" in plain Markdown.
+- The change log should appear in the CHAT, NOT inside the preview code.
+- Example format:
+[/PREVIEW]
+
+**Change Log:**
+- Added feature X
+- Modified component Y
+- Fixed issue Z
 
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