refactor(tinychat): improve download progress polling mechanism

[li-linfeng]

• Add exponential backoff retry strategy for download progress polling
• Replace interval-based polling with while loop and dynamic intervals
• Add isPolling flag to prevent concurrent polling
• Add allProgressComplete flag for better completion tracking
• Remove downloadProgressInterval as it's no longer needed
• Optimize polling termination conditions

The changes make the download progress tracking more efficient and reliable by:

1. Using exponential backoff (1s to 32s) when no progress is detected
2. Automatically stopping polling after max retries or completion
3. Preventing multiple concurrent polling operations