feat: download and decrypt inbound WeChat CDN images

[yuxianzhao0921-del]

Summary

Upstream had AES-128-ECB helpers and a downloadAndDecryptMedia function but never wired them into the polling loop for inbound images. This PR closes that gap.

What's included

• Robust parseAesKey: WeChat's inbound media.aes_key is base64-encoded hex string (32 UTF-8 chars → 16 key bytes), not raw base64 of 16 bytes. The new parser handles both formats gracefully.
• downloadAndSaveImage: download → decrypt → detect format via magic bytes (JPEG/PNG/GIF/WebP) → save to ~/.claude/channels/wechat/images/
• Expanded ImageItem interface: added aeskey (hex), media.full_url, and CDNMedia type to match actual inbound message structure
• Wired into polling loop: image messages now deliver a file path ([图片: /path/to/file.jpg]) instead of just [图片]

How it works

WeChat CDN image → fetch encrypted blob → AES-128-ECB decrypt → magic bytes detect format → save to disk → pass path to Claude

Relation to PR #10

This overlaps partially with #10. Key differences:

• This PR uses cdn_url / media.full_url directly; feat: receive media messages (image, video, voice, file) #10 uses encrypt_query_param + CDN base URL
• This PR adds robust dual-format AES key parsing that handles edge cases
• This PR includes magic-bytes format detection (PNG/GIF/WebP/JPEG)
• This PR is image-only; feat: receive media messages (image, video, voice, file) #10 covers video/voice/file as well

Both approaches work. Happy to collaborate on merging the best parts if needed.

Test plan

• Tested with real WeChat CDN images (AES-128-ECB encrypted JPEG)
• Verified Claude can read the saved image via the Read tool
• Confirmed backward compatibility — text messages unaffected

🤖 Generated with Claude Code