Show pasted image previews on remote threads

[SDSLeon]

• Pasted images on remote threads are uploaded to the host desktop, but composer previews tried to read the local path and showed broken images; previews now render from a local object URL of the pasted bytes.
• Centralized image attachment URL resolution in attachmentImageUrl(...), preferring the pasted-bytes preview, then the paired desktop's image endpoint, then the local-file protocol.
• Remote thread and remote draft composers now resolve previews and the lightbox through the desktop's image endpoint instead of assuming a local path.
• Remote threads now save pasted clipboard images to the host desktop so the agent running there can access them.
• Revoke paste object URLs when attachments are removed or cleared to avoid leaks; added unit coverage for preview resolution, revocation, and remote saving.