How to extend (or kern) a subtitle to the text width?

Question

I have a subtitle that is almost as large as the text width.  For aesthetical purposes, I would like to strech it a bit so it covers the whole text width, and be more harmonious with the picture below the subtitle.  How can I achieve this trick?
Here's a code that show the subtitle:
documentclass[letterpaper]{book}
usepackage[T1]{fontenc}
usepackage{microtype}
usepackage{lmodern}
usepackage[utf8]{inputenc}
usepackage[french]{babel}
usepackage{anysize}
marginsize{1.5in}{1in}{0.5in}{0.5in}
usepackage[x11names]{xcolor}
definecolor{backgroundcolor}{RGB}{180, 180, 160}
definecolor{subtitle}{RGB}{240, 240, 200}
usepackage{graphicx}
usepackage{here}
usepackage{afterpage}

usepackage{librebaskerville}
usepackage{tikz}
usetikzlibrary{calc}

newcommand*{maintitle}{begingroup

pagecolor{backgroundcolor}

centering

flushleft{
        textcolor{subtitle}{Hugetextsf{La théorie classique des champs relativistes}}
    }

begin{figure}[H]
        center
        includegraphics[width=1textwidth]{example-image}
    end{figure}

endgroup}

begin{document}

begin{titlepage}

% FRAME AROUND PAGE :
    begin{tikzpicture}[remember picture, overlay]
        draw[line width = 1in,color = white] ($(current page.north west) + (0.25in,0.25in)$) rectangle ($(current page.south east) + (0.25in,-0.25in)$);
        draw[line width = 0.25pt,color = black] ($(current page.north west) + (0.75in,-0.25in)$) rectangle ($(current page.south east) + (-0.25in,0.25in)$);
        draw[line width = 1pt,color = white] ($(current page.north west) + (1in,-0.5in)$) rectangle ($(current page.south east) + (-0.5in,0.5in)$);
    end{tikzpicture}

maintitle
end{titlepage}

end{document}

Preview of what this code is doing:

All I need here is to extend or stretch the subtitle so it has a lenght equal to the textwidth.  The space to the right should be the same as the space on the left side (which should stay the same).

EDIT:  I've updated the code above.  Maybe I wasn't clear about how much stretching I wanted.  I certainly don't want the subtitle to be stretched right to the vertical lines on both sides.  The subtitle in flushed left, and there's a nice space between the left line and the beginning of the subtitle.  This should stay fixed.  But then, I need to stretch the right part just a bit, to get a subtitle of the same size as the picture below it.

egreg · Answer

I'd avoid this.
Anyway, here's how you can do it. Note that accented characters and possible ligatures have to be braced.
I made some adjustments:

anysize is much less powerful than geometry and has not been maintained in 26 years;
the here package is obsolete; use float instead, but never use [H];
the minipage doesn't seem needed, particularly with width 2textwidth;
to set something larger than the text width, use makebox;
I added some space at either size so the text doesn't touch the frame lines.

documentclass[letterpaper]{book}
usepackage[inner=1.5in,outer=1in,top=0.5in,bottom=0.5in]{geometry}
%usepackage[utf8]{inputenc} % not needed with recent versions of LaTeX
usepackage[T1]{fontenc}
usepackage[french]{babel}
usepackage{microtype}
usepackage{graphicx}
usepackage[x11names]{xcolor}
definecolor{backgroundcolor}{RGB}{180, 180, 160}
definecolor{subtitle}{RGB}{240, 240, 200}

usepackage{lmodern} % for sans serif
usepackage{librebaskerville}
usepackage{tikz}
usetikzlibrary{calc}

usepackage{xparse}

ExplSyntaxOn
NewDocumentCommand{expandto}{mm}
 {
  seq_set_split:Nnn l_tmpa_seq { ~ } { #2 }
  seq_set_map:NNn l_tmpb_seq l_tmpa_seq { cham_expand_word:n { ##1 } }
  leavevmode
  hbox_to_wd:nn { #1 }
   {
    hspace{3pt}
    seq_use:Nn l_tmpb_seq { hspace{0.33333em plus 1fil} }
    hspace{3pt}
   }
 }
cs_new_protected:Nn cham_expand_word:n
 {
  seq_set_split:Nnn l_tmpa_seq { } { #1 }
  seq_use:Nn l_tmpa_seq { hspace{0pt plus 0.6fil} }
 }
ExplSyntaxOff

newcommand*{maintitle}{%
  begingroup
  pagecolor{backgroundcolor}
  centering
  makebox[textwidth]{%
    color{subtitle}Hugesffamily
    expandto{textwidth+1in}{La th{é}orie classique des champs relativistes}%
  }%
  endgroup
}

begin{document}

begin{titlepage}

% FRAME AROUND PAGE :
begin{tikzpicture}[remember picture, overlay]
  draw[line width = 1in,color = white] ($(current page.north west) + (0.25in,0.25in)$) rectangle ($(current page.south east) + (0.25in,-0.25in)$);
  draw[line width = 0.25pt,color = black] ($(current page.north west) + (0.75in,-0.25in)$) rectangle ($(current page.south east) + (-0.25in,0.25in)$);
  draw[line width = 1pt,color = white] ($(current page.north west) + (1in,-0.5in)$) rectangle ($(current page.south east) + (-0.5in,0.5in)$);
end{tikzpicture}

maintitle

end{titlepage}

end{document}

Look at the picture and see why I started with ‘I'd avoid this’. ;-)
If the title should use the text width:
documentclass[letterpaper]{book}
usepackage[inner=1.5in,outer=1in,top=0.5in,bottom=0.5in]{geometry}
%usepackage[utf8]{inputenc} % not needed with recent versions of LaTeX
usepackage[T1]{fontenc}
usepackage[french]{babel}
usepackage{microtype}
usepackage{graphicx}
usepackage[x11names]{xcolor}
definecolor{backgroundcolor}{RGB}{180, 180, 160}
definecolor{subtitle}{RGB}{240, 240, 200}

usepackage{lmodern} % for sans serif
usepackage{librebaskerville}
usepackage{tikz}
usetikzlibrary{calc}

usepackage{xparse}

ExplSyntaxOn
NewDocumentCommand{expandto}{O{0pt}mm}
 {
  seq_set_split:Nnn l_tmpa_seq { ~ } { #3 }
  seq_set_map:NNn l_tmpb_seq l_tmpa_seq { cham_expand_word:n { ##1 } }
  leavevmode
  hbox_to_wd:nn { #2 }
   {
    hspace{#1}
    seq_use:Nn l_tmpb_seq { hspace{0.33333em plus 1fil} }
    hspace{#1}
   }
 }
cs_new_protected:Nn cham_expand_word:n
 {
  seq_set_split:Nnn l_tmpa_seq { } { #1 }
  seq_use:Nn l_tmpa_seq { hspace{0pt plus 0.6fil} }
 }
ExplSyntaxOff

newcommand*{maintitle}{%
  begingroup
  pagecolor{backgroundcolor}
  centering
  makebox[textwidth]{%
    color{subtitle}Hugesffamily
    expandto{textwidth}{La th{é}orie classique des champs relativistes}%
  }
  includegraphics[width=textwidth]{example-image}%
  endgroup
}

begin{document}

begin{titlepage}

% FRAME AROUND PAGE :
begin{tikzpicture}[remember picture, overlay]
  draw[line width = 1in,color = white] ($(current page.north west) + (0.25in,0.25in)$) rectangle ($(current page.south east) + (0.25in,-0.25in)$);
  draw[line width = 0.25pt,color = black] ($(current page.north west) + (0.75in,-0.25in)$) rectangle ($(current page.south east) + (-0.25in,0.25in)$);
  draw[line width = 1pt,color = white] ($(current page.north west) + (1in,-0.5in)$) rectangle ($(current page.south east) + (-0.5in,0.5in)$);
end{tikzpicture}

maintitle

end{titlepage}

end{document}

About your edit:

flushleft is not a user level command and it takes no argument;

there is no need to use figure around an image.

I transformed the padding on either side into an optional argument to expandto just in case you need it elsewhere.

How to extend (or kern) a subtitle to the text width?

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