Use a pre-populated node_modules tree instead of running `npm ci` at build time.

The web app is bundled by webpack, which upstream drives through
BuildWebApp.cmake: it stages src/app and src/web-app into the build tree and
then runs `npm ci --ignore-scripts` there.  That step needs the network, which
a distribution build does not have.  Gentoo ships the dependency tree as a
separate distfile generated from this release's package-lock.json and points
LEMONADE_PREBUILT_NODE_MODULES at it, so the install step can be skipped and
`npm run build` runs fully offline.

Not sent upstream: this is a packaging hook rather than a bug fix.
--- a/src/web-app/BuildWebApp.cmake
+++ b/src/web-app/BuildWebApp.cmake
@@ -65,6 +65,27 @@
 # the staged web-app directory, which now sits at staging/web-app/.
 set(WEB_APP_BUILD_SOURCE_DIR "${STAGED_WEB_APP_DIR}")
 
+# Distribution builds have no network access at build time, so `npm ci` cannot
+# run. When LEMONADE_PREBUILT_NODE_MODULES points at a node_modules tree that
+# was populated ahead of time from this release's package-lock.json, link it
+# into the freshly staged web-app directory and skip the install step below.
+if(NOT "$ENV{LEMONADE_PREBUILT_NODE_MODULES}" STREQUAL "")
+    if(NOT EXISTS "$ENV{LEMONADE_PREBUILT_NODE_MODULES}")
+        message(FATAL_ERROR
+            "LEMONADE_PREBUILT_NODE_MODULES is set to $ENV{LEMONADE_PREBUILT_NODE_MODULES} but that directory does not exist")
+    endif()
+    message(STATUS "Using pre-populated node_modules from $ENV{LEMONADE_PREBUILT_NODE_MODULES}")
+    execute_process(
+        COMMAND "${CMAKE_COMMAND}" -E create_symlink
+            "$ENV{LEMONADE_PREBUILT_NODE_MODULES}"
+            "${WEB_APP_BUILD_SOURCE_DIR}/node_modules"
+        RESULT_VARIABLE LINK_RESULT
+    )
+    if(NOT LINK_RESULT EQUAL 0)
+        message(FATAL_ERROR "Failed to link pre-populated node_modules (exit code ${LINK_RESULT})")
+    endif()
+endif()
+
 # System nodejs modules and KaTeX fonts integration
 if(USE_SYSTEM_NODEJS_MODULES_ENABLED)
     set(SYSTEM_NODE_MODULES "/usr/share/nodejs")
@@ -105,8 +126,9 @@
     find_program(WEBPACK_EXECUTABLE webpack)
 endif()
 
-# Install dependencies only in non-system mode
-if(NOT USE_SYSTEM_NODEJS_MODULES_ENABLED)
+# Install dependencies only in non-system mode, and only when the staged tree
+# does not already carry a node_modules (see LEMONADE_PREBUILT_NODE_MODULES).
+if(NOT USE_SYSTEM_NODEJS_MODULES_ENABLED AND NOT EXISTS "${WEB_APP_BUILD_SOURCE_DIR}/node_modules")
     if(NOT NPM_EXECUTABLE)
         message(FATAL_ERROR "npm not found - non-system web app build requires npm")
     endif()