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/browser/dto/exec.dto.ts

@@ -0,0 +1,30 @@
+import { ApiProperty, ApiPropertyOptional } from "@nestjs/swagger";
+import { IsOptional, IsString, IsUrl } from "class-validator";
+
+export class ExecDto {
+  @ApiPropertyOptional({
+    description:
+      "Session name previously created by POST /login. If omitted, executes without a stored session.",
+    example: "test-session-1",
+  })
+  @IsOptional()
+  @IsString()
+  sessionName?: string;
+
+  @ApiPropertyOptional({
+    description:
+      "URL to navigate to before executing code. Skipped if omitted.",
+    example: "https://cabinet-liquio-diia-stg.kitsoft.ua/messages",
+  })
+  @IsOptional()
+  @IsUrl({ require_tld: true, require_protocol: true })
+  url?: string;
+
+  @ApiProperty({
+    description:
+      "JavaScript code to execute. Receives `page` (Playwright Page) and `context` (BrowserContext) as arguments. May be async. Return value is serialised and returned.",
+    example: "return await page.title();",
+  })
+  @IsString()
+  code: string;
+}