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/domain/design/agent.go

@@ -18,6 +18,7 @@ type AgentContent struct {
 	Text        string
 	TextSource  string
 	ImageURL    string
+	ImageName   string
 	ImageWidth  int64
 	ImageHeight int64
 }