Run CI on macOS

The scaffold copied margin-calendar's workflow, which builds for Linux too. This app is
macOS only, and two tests say so: trash_does_not_hard_delete asserts $HOME/.Trash, and
fits.test.ts enumerates chords from the primary modifier the user agent implies.

.github/workflows/ci.yml

@@ -11,7 +11,10 @@ concurrency:
 
 jobs:
   frontend:
-    runs-on: ubuntu-latest
+    # macOS, not Linux. The keymap layer resolves the primary modifier from the user agent, so on a
+    # Linux runner `fits.test.ts` enumerates Ctrl chords and compares them against the Command ones
+    # the app actually binds. The app is macOS only; CI runs where it ships.
+    runs-on: macos-latest
     steps:
       - uses: actions/checkout@v7
 
@@ -31,23 +34,12 @@ jobs:
       - run: pnpm test
 
   rust:
-    runs-on: ubuntu-22.04
+    # Same reason, more sharply: `trash_does_not_hard_delete` asserts the file lands in
+    # `$HOME/.Trash`, which is where macOS puts it and not where Linux does.
+    runs-on: macos-latest
     steps:
       - uses: actions/checkout@v7
 
-      - name: Install Linux dependencies
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y \
-            libwebkit2gtk-4.1-dev \
-            libgtk-3-dev \
-            libayatana-appindicator3-dev \
-            librsvg2-dev \
-            patchelf \
-            libxdo-dev \
-            libssl-dev \
-            build-essential
-
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable