Standardize warning output to use style.PrintWarning (gt-g6kor)

- Add PrintWarning helper in internal/style/style.go
- Update 35 warning message outputs across 16 files to use consistent format
- All warnings now display as "⚠ Warning: <message>" in yellow/bold

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/molecule_attach_from_mail.go

@@ -111,7 +111,7 @@ func runMoleculeAttachFromMail(cmd *cobra.Command, args []string) error {
 	// Mark mail as read
 	if err := mailbox.MarkRead(mailID); err != nil {
 		// Non-fatal: log warning but don't fail
-		fmt.Fprintf(os.Stderr, "Warning: could not mark mail as read: %v\n", err)
+		style.PrintWarning("could not mark mail as read: %v", err)
 	}
 
 	// Output success