feat: Add shells for building the Linux app

Add shells for building the Collabora Office Linux app.

coda-online.nix is the online shell with the additional qt packages to
build the desktop app.

coda-core.nix is th exact same as core.nix; currently you can even use
the same build of core for both browser and desktop. However, this shell
exists to allow for later changes that would make the core builds
incompatible.

Author: celestial-starfruit

shells/coda-core.nix

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: 2025 Collabora Productivity Limited
+#
+# SPDX-License-Identifier: MIT
+
+import ./core.nix

shells/coda-online.nix

@@ -0,0 +1,29 @@
+# SPDX-FileCopyrightText: 2025 Collabora Productivity Limited
+#
+# SPDX-License-Identifier: MIT
+
+{ config }:
+{
+  kdePackages,
+  stdenv,
+  ...
+}:
+config.shells.online.result.x86_64-linux.overrideAttrs (old: {
+   nativeBuildInputs = old.nativeBuildInputs ++ [
+    kdePackages.qtbase.dev
+    kdePackages.qttools
+    (stdenv.mkDerivation {
+      name = "qtlibexec";
+
+      src = kdePackages.qtbase;
+
+      buildPhase = ''
+        mkdir -p $out
+        ln -s ${kdePackages.qtbase}/libexec $out/bin
+      '';
+    })
+    kdePackages.qtbase
+    kdePackages.qtwebengine
+    kdePackages.wrapQtAppsHook
+  ];
+})