Vendor Nunito as a TTF so the device renders in the right face

Slint cannot read woff2, which is the only form web/public/fonts/ carries, and Raspbian
has no fonts-nunito to install instead - so the device was going to fall back to
DejaVu Sans, which is not what any of this was drawn in. The upstream variable TTF
(SIL OFL, weights 200-1000) is small enough to vendor and covers every weight the
design uses, including the 800/900 the headings lean on.

SLINT_DEFAULT_FONT takes one file and makes it the primary font. shell.nix now points
at this copy rather than at nixpkgs' own, so development and the device render from the
identical file, and the binary falls back to finding it beside the executable when
neither the dev shell nor the device's launcher has set the variable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

slint-frontend/shell.nix

@@ -33,12 +33,10 @@ pkgs.mkShell {
     python3
   ];
 
-  # Nunito is the face the design is drawn in. The web front-end self-hosts it as
-  # woff2, which Slint cannot read, so the dev shell points Slint at the TTF instead
-  # (`SLINT_DEFAULT_FONT` takes one file and makes it the primary font). Without it
-  # everything still renders, in whatever sans fontconfig hands back.
-  SLINT_DEFAULT_FONT =
-    "${pkgs.nunito}/share/fonts/truetype/Nunito/Nunito[wght].ttf";
+  # Nunito is the face the design is drawn in. `SLINT_DEFAULT_FONT` takes one file and
+  # makes it the primary font. This points at the copy vendored in `fonts/` rather than
+  # at nixpkgs, so development and the device render from the identical file.
+  SLINT_DEFAULT_FONT = toString ./fonts/Nunito.ttf;
 
   buildInputs = runtimeLibs ++ (with pkgs; [
     fontconfig