ci: run node 24

The frontend job was pinned to node 16, which is old enough that some of
the packages patched in this branch refuse to install there. tar and
esbuild want node 18 or newer, serialize-javascript wants 20.

Node was also only pinned in the frontend job. The backend,
proto-compile-check and itest jobs all reference matrix.node_version
without declaring it in their matrix, so the expression resolved to an
empty string and setup-node fell back to whatever the runner happened to
ship. Two of those run `yarn install --frozen-lockfile`, so they need a
known version too.

Both are fixed by a NODE_VERSION env var, matching how GO_VERSION is
already handled. It replaces the matrix entry rather than feeding it,
because the env context isn't available inside strategy.matrix.

.github/workflows/main.yml

@@ -26,6 +26,11 @@ env:
   # /dev.Dockerfile
   GO_VERSION: 1.25.11
 
+  # Only applies to the CI jobs below. The note above covers GO_VERSION alone:
+  # /Dockerfile and /dev.Dockerfile pin their own Node version for release
+  # builds, and it does not have to match this one.
+  NODE_VERSION: 24.x
+
 jobs:
   ########################
   # frontend build checks
@@ -36,7 +41,6 @@ jobs:
 
     strategy:
       matrix:
-        node_version: [ 16.x ]
         os: [ ubuntu-latest, windows-latest, macOS-latest ]
 
     steps:
@@ -45,10 +49,10 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: setup nodejs ${{ matrix.node_version }}
+      - name: setup nodejs ${{ env.NODE_VERSION }}
         uses: ./.github/actions/setup-node
         with:
-          node-version: '${{ matrix.node_version }}'
+          node-version: '${{ env.NODE_VERSION }}'
 
       - name: install dependencies
         working-directory: ./app
@@ -84,10 +88,10 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: setup nodejs ${{ matrix.node_version }}
+      - name: setup nodejs ${{ env.NODE_VERSION }}
         uses: ./.github/actions/setup-node
         with:
-          node-version: '${{ matrix.node_version }}'
+          node-version: '${{ env.NODE_VERSION }}'
 
       - name: setup go ${{ env.GO_VERSION }}
         uses: ./.github/actions/setup-go
@@ -144,10 +148,10 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: setup nodejs ${{ matrix.node_version }}
+      - name: setup nodejs ${{ env.NODE_VERSION }}
         uses: ./.github/actions/setup-node
         with:
-          node-version: '${{ matrix.node_version }}'
+          node-version: '${{ env.NODE_VERSION }}'
 
       - name: setup go ${{ env.GO_VERSION }}
         uses: ./.github/actions/setup-go
@@ -324,10 +328,10 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: setup nodejs ${{ matrix.node_version }}
+      - name: setup nodejs ${{ env.NODE_VERSION }}
         uses: ./.github/actions/setup-node
         with:
-          node-version: '${{ matrix.node_version }}'
+          node-version: '${{ env.NODE_VERSION }}'
 
       - name: setup go ${{ env.GO_VERSION }}
         uses: ./.github/actions/setup-go