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/types/types.go

@@ -49,6 +49,7 @@ type AgentContent struct {
 	Text        string `json:"text,optional"`
 	TextSource  string `json:"textSource,optional"`
 	ImageUrl    string `json:"imageUrl,optional"`
+	ImageName   string `json:"imageName,optional"`
 	ImageWidth  int64  `json:"imageWidth,optional"`
 	ImageHeight int64  `json:"imageHeight,optional"`
 }