fdroid-publish: add silvermail (uk.silverlabs.silvermail, pinned release cert)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -18,7 +18,7 @@ lock), then read the published index back.
 ```yaml
 - uses: https://git.silverlabs.uk/SilverLABS/ci-actions/fdroid-publish@v1
   with:
-    app: silverchat            # silverchat | silvervpn | silverstore
+    app: silverchat            # silverchat | silvervpn | silverstore | silvermail
     apk: path/to/app-release.apk
     channel: beta              # default
     version-code: ${{ github.run_number }}   # optional; read from the APK if omitted

fdroid-publish/action.yml

@@ -7,7 +7,7 @@ description: >
   reads the index back afterwards.
 inputs:
   app:
-    description: silverchat | silvervpn | silverstore
+    description: silverchat | silvervpn | silverstore | silvermail
     required: true
   apk:
     description: Path to the signed APK

fdroid-publish/publish.sh

@@ -30,7 +30,8 @@ case "$app" in
   silverchat)  pkg=uk.silverlabs.silverchat;  cert=06c4ac92e17594d94d75eb9ed85c619aa3aa73cee3d9e64c12ab11255d28a280 ;;
   silvervpn)   pkg=uk.silverlabs.silvervpn;   cert=706a56b202b37d5fb2f5d4e89dea819fa53a372eecc31999a125782e6a3c8cab ;;
   silverstore) pkg=uk.silverlabs.silverstore; cert=f3d835bbd3717f151400b4e0e95212289d585f568690fb05634881f26ec251e4 ;;
-  *) fail "app must be silverchat, silvervpn or silverstore (got '$app')" ;;
+  silvermail)  pkg=uk.silverlabs.silvermail;  cert=ccb60c6adc9b015cef88ad9a18a6194178c52754097a3f1d61029b1f6ef32ba2 ;;
+  *) fail "app must be silverchat, silvervpn, silverstore or silvermail (got '$app')" ;;
 esac
 case "$channel" in
   beta)   root=/srv/fdroid-test; url=https://apps.silverlined.uk/fdroid-test/repo ;;