fix(ci): install libzim-dev so native @openzim/libzim build can resolve headers

The Build Admin workflow's `npm ci` step started failing on every PR opened
since early May 2026 with:

    fatal error: zim/archive.h: No such file or directory
    #include <zim/archive.h>

@openzim/libzim is a native Node module that compiles against the system
libzim C++ library via node-gyp. Builds were green through 2026-05-04 because
the ubuntu-latest runner image included libzim-dev preinstalled. Sometime
between 2026-05-04 and 2026-05-05 the runner image stopped shipping it, and
every admin PR since has failed at the libzim compile step.

Cuyua9's #733 (kiwix self-heal) and chriscrosstalk's #832 (vendor-aware AMD
HSA override) both hit this at 14:16 UTC and 16:01 UTC on 2026-05-05.

Fix: add an apt step that installs libzim-dev before `npm ci` runs.

.github/workflows/build-admin-on-pr.yml

@@ -16,6 +16,12 @@ jobs:
           node-version: '24'
           cache: 'npm'
 
+      - name: Install libzim system headers
+        # @openzim/libzim is a native Node module that compiles against the system
+        # libzim C++ library via node-gyp. The ubuntu-latest runner image stopped
+        # shipping libzim-dev preinstalled in early May 2026, breaking npm ci.
+        run: sudo apt-get update && sudo apt-get install -y libzim-dev
+
       - name: Install dependencies
         run: npm ci
         working-directory: ./admin