feat(linux): port to gtk4

.changes/libadwaita-feature.md

@@ -0,0 +1,5 @@
+---
+"tao": minor
+---
+
+Add optional `libadwaita` feature for GTK4 Linux builds. When enabled, uses `adw::Application` instead of `gtk::Application` for proper GNOME HIG styling (rounded corners, themed menus, etc.).

.changes/linux-gtk4.md

@@ -0,0 +1,5 @@
+---
+"tao": minor
+---
+
+Port the Linux backend from GTK3 to GTK4 (`gtk4` 0.11, `glib` 0.22). Adds an optional `libadwaita` feature. Public gtk types exposed through the Unix extension traits are now their GTK4 equivalents, and the minimum supported Rust version rises to 1.92 and the minimum system GTK to 4.10 (both required by gtk4-rs 0.11 and the `Accessible` interface the window type implements).

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
         if: matrix.platform.os == 'ubuntu-latest'
         run: |
           sudo apt-get update
-          sudo apt-get install -y libgtk-3-dev
+          sudo apt-get install -y libgtk-4-dev libdbus-1-dev
 
       - name: Install GCC Multilib
         if: (matrix.platform.os == 'ubuntu-latest') && contains(matrix.platform.target, 'i686')
@@ -63,8 +63,9 @@ jobs:
         if: contains(matrix.platform.target, 'android')
         run: cargo install cargo-apk
 
-      - uses: dtolnay/rust-toolchain@1.74
+      - uses: dtolnay/rust-toolchain@master
         with:
+          toolchain: "1.92"
           targets: ${{ matrix.platform.target }}
 
       - name: Check documentation