feat: make devbox.json cross-platform for GTK4/libadwaita

Linux now gets gtk4/libadwaita declared as nix devbox packages (scoped
to linux platforms, with the dev output requested explicitly - nixpkgs
splits runtime libs from headers/pkgconfig, and devbox only installs
the default output unless told otherwise). macOS keeps using Homebrew's
build exactly as before (nixpkgs' GTK build is much rockier there).

PKG_CONFIG_PATH moved out of the static env block (which can't be
platform-conditional) into init_hook, which can branch on uname.
Verified building clean with 0 warnings on both this machine (Linux)
and apfel (macOS).

devbox.json

@@ -1,18 +1,29 @@
 {
   "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.17.2/.schema/devbox.schema.json",
-  "packages": [
-    "rustc@latest",
-    "cargo@latest",
-    "pkg-config@latest",
-    "rustfmt@latest"
-  ],
+  "packages": {
+    "rustc": "latest",
+    "cargo": "latest",
+    "pkg-config": "latest",
+    "rustfmt": "latest",
+    "gtk4": {
+      "version": "latest",
+      "platforms": ["x86_64-linux", "aarch64-linux"],
+      "outputs": ["out", "dev"]
+    },
+    "libadwaita": {
+      "version": "latest",
+      "platforms": ["x86_64-linux", "aarch64-linux"],
+      "outputs": ["out", "dev"]
+    }
+  },
   "env": {
-    "PATH": "/opt/homebrew/bin:/opt/homebrew/sbin:$PATH",
-    "PKG_CONFIG_PATH": "/opt/homebrew/lib/pkgconfig",
-    "PKG_CONFIG_PATH_arm64_apple_darwin": "/opt/homebrew/lib/pkgconfig"
+    "PATH": "/opt/homebrew/bin:/opt/homebrew/sbin:$PATH"
   },
   "shell": {
-    "init_hook": [],
+    "init_hook": [
+      "export PKG_CONFIG_PATH=\"$DEVBOX_PACKAGES_DIR/lib/pkgconfig:$PKG_CONFIG_PATH\"",
+      "if [[ \"$(uname)\" == \"Darwin\" ]]; then export PKG_CONFIG_PATH=\"/opt/homebrew/lib/pkgconfig:$PKG_CONFIG_PATH\"; export PKG_CONFIG_PATH_arm64_apple_darwin=\"/opt/homebrew/lib/pkgconfig\"; fi"
+    ],
     "scripts": {
       "build": [
         "cargo build --release"