feat(composer): carry reference image names and submit on Enter

Propagate an optional imageName through the agent content pipeline
(frontend types → API → domain → prompt building) so reference images
render with their name instead of a bare "image" label.

Also switch PromptComposer to submit on plain Enter (Shift+Enter for
newline, IME-safe) and add a submitDisabled guard wired from the canvas
and home composers.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/logic/mapper.go

@@ -519,6 +519,7 @@ func toDomainAgentChat(req *types.AgentChatRequest) design.AgentChatRequest {
 				Text:        content.Text,
 				TextSource:  content.TextSource,
 				ImageURL:    content.ImageUrl,
+				ImageName:   content.ImageName,
 				ImageWidth:  content.ImageWidth,
 				ImageHeight: content.ImageHeight,
 			})