feat(session): add per-message created_at timestamps

- Persistence layer (jsonl.go addMsg/SetHistory) normalizes CreatedAt
  when missing so the invariant is guaranteed at the storage boundary
- API layer (session.go) exposes created_at on all transcript message
  types with session.updated fallback for legacy messages
- Frontend uses per-message timestamps when available
- messagesContentEqual ignores CreatedAt for tail-matching after
  JSONL roundtrip

Fixes #2787

pkg/providers/protocoltypes/types.go

@@ -1,5 +1,7 @@
 package protocoltypes
 
+import "time"
+
 type ToolCall struct {
 	ID               string         `json:"id"`
 	Type             string         `json:"type,omitempty"`
@@ -87,6 +89,7 @@ type Message struct {
 	Role             string         `json:"role"`
 	Content          string         `json:"content"`
 	ModelName        string         `json:"model_name,omitempty"`
+	CreatedAt        *time.Time     `json:"created_at,omitempty"`
 	Media            []string       `json:"media,omitempty"`
 	Attachments      []Attachment   `json:"attachments,omitempty"`
 	ReasoningContent string         `json:"reasoning_content,omitempty"`