Fix Electron not opening: run npm install on core + check DISPLAY

- core.sh: run npm install after cloning (downloads Electron binary
  into app/node_modules — required for the UI to launch)
- start.sh template: check if app/node_modules exists and run
  npm install on first start if missing
- start.sh template: warn if DISPLAY/WAYLAND_DISPLAY not set

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/core.sh

@@ -24,5 +24,8 @@ setup_core() {
     spin "Cloning context-studio-core..." \
         git clone "$CS_CORE_REPO" "$CS_CORE_DIR" \
         || die "Failed to clone context-studio-core. Check your SSH key and network."
+    spin "Installing core dependencies (npm install)..." \
+        bash -c "cd '$CS_CORE_DIR' && npm install" \
+        || die "npm install failed for context-studio-core."
     success "Core installed at $CS_CORE_DIR"
 }