BridgePlatform was missing SendImage and SendFile methods, causing
`cc-connect send --image/--file` to silently succeed without delivering
content through the bridge protocol.
Add SendImage/SendFile following the existing SendAudio pattern (check
adapter capability, base64-encode data, send via sendToAdapter), plus
compile-time interface checks and protocol documentation for the new
`image` and `file` message types.
Closes#708
Co-authored-by: Claude <noreply@anthropic.com>