Add containerized mock server for local dev

Introduces a TypeScript-based Express server with multi-stage Docker build
Facilitates local testing and includes basic endpoints and usage instructions

Author: mbertelsen

server/server/Dockerfile

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+# Dockerfile for editor mock API (TypeScript/Express)
+FROM node:18-alpine AS build
+WORKDIR /app
+# Copy only package.json from the build context. The repo root package-lock.json
+# is not available when building from the subfolder context, which caused
+# BuildKit checksum errors. Use npm ci when a lockfile exists, otherwise
+# fall back to npm install so the build doesn't fail.
+COPY package.json ./
+RUN npm ci --production=false || npm install --production=false
+COPY tsconfig.json ./
+COPY src ./src
+RUN npm run build
+
+FROM node:18-alpine
+WORKDIR /app
+# Install only production deps
+COPY package.json ./
+# If a package-lock.json is present, npm ci will be used; if not, fall back
+# to npm install so the build doesn't fail when the lockfile is absent in the
+# build context (common when CI checkouts or subfolders don't include it).
+RUN npm ci --production || npm install --production
+COPY --from=build /app/dist ./dist
+EXPOSE 4000
+CMD ["node", "dist/index.js"]

server/server/README.md

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+Editor mock server for local development (canonical server for this repo)
+
+Run:
+
+```powershell
+cd server/server
+npm install
+npm run dev
+```
+
+Endpoints:
+- GET /health
+- GET /me
+- POST /logout

server/server/package.json

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+{
+  "name": "app-scripting-editor-server",
+  "version": "0.1.0",
+  "private": true,
+  "main": "dist/index.js",
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "start": "node dist/index.js",
+    "dev": "ts-node-dev --respawn --transpile-only src/index.ts"
+  },
+  "dependencies": {
+    "cors": "^2.8.5",
+    "express": "^4.18.2"
+  },
+  "devDependencies": {
+    "@types/express": "^4.17.17",
+    "@types/cors": "^2.8.12",
+    "@types/node": "^18.0.0",
+    "ts-node-dev": "^2.0.0",
+    "typescript": "^5.0.4"
+  }
+}