chore(repo): restructure as monorepo with server and client workspaces

- move NestJS app into server/ subdirectory
- add client/ React+TypeScript (Vite) app with Hello World
- update docker-compose to build and run both services
- add root package.json declaring npm workspaces
- update .gitignore to cover node_modules and dist at all depths

server/src/config/app.config.ts

@@ -0,0 +1,46 @@
+import { IsInt, IsString, Min, Max } from "class-validator";
+import { plainToInstance } from "class-transformer";
+import { validateSync } from "class-validator";
+
+import pkg from "../../package.json";
+
+export class AppConfig {
+  @IsString()
+  NODE_ENV: string = "development";
+
+  @IsInt()
+  @Min(1)
+  @Max(65535)
+  PORT: number = 3000;
+
+  @IsString()
+  APP_NAME: string = pkg.name;
+
+  @IsString()
+  APP_VERSION: string = pkg.version;
+
+  @IsString()
+  KEYS_DIR: string = "keys";
+
+  @IsString()
+  DB_PATH: string = "data/sessions.db";
+
+  @IsInt()
+  @Min(1)
+  SESSION_IDLE_TIMEOUT_MINUTES: number = 30;
+
+  @IsInt()
+  @Min(1)
+  SESSION_DELETE_CLOSED_DAYS: number = 7;
+}
+
+export function validateAppConfig(config: Record<string, unknown>): AppConfig {
+  const validated = plainToInstance(AppConfig, config, {
+    enableImplicitConversion: true,
+  });
+  const errors = validateSync(validated, { skipMissingProperties: false });
+  if (errors.length > 0) {
+    throw new Error(errors.toString());
+  }
+  return validated;
+}