Support FileCaption fallback for grouped media downloads

[expary]

Summary

• Apply the first non-empty caption in a grouped message as a fallback for group members that do not have their own caption.
• Keep existing single-message behavior and preserve per-file captions when present.
• Document the FileCaption behavior for grouped media templates.

Tests

• go test ./app/dl
• go test ./app/dl ./app/chat ./pkg/tmessage
• go test $(go list ./... | grep -v '^github.com/iyear/tdl/test$')
• go test ./... (from core/)
• go test ./... (from extension/)

Closes #1208