webui: preserve partial content when streaming errors occur

[Aaryan-549]

Summary

Fixes #13709 - WebUI now preserves partial content when streaming errors occur instead of discarding the entire message.

Changes

• Enhanced onChunk callback in ChatScreen.tsx to handle streaming errors
• Added user-friendly toast notifications for different error scenarios
• Preserves partial response content when connection is interrupted
• Added try-catch blocks around message sending functions

Behavior

Before: When server is killed during generation (Ctrl+C), the entire message disappears
After: Partial content is preserved with a clear error indicator, similar to ChatGPT

Testing

• Tested with server interruption during generation
• Verified partial content is preserved
• Error messages display correctly
• No regression in normal operation

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

[ngxson]

custom style is redundant and can cause problem with themes