chore(frontend): move from yarn1 to pnpm (#10072)

## 🧢 Overview
This PR migrates the AutoGPT Platform frontend from [yarn
1](https://classic.yarnpkg.com/lang/en/) to [pnpm](https://pnpm.io/)
using **corepack** for automatic package manager management.

**yarn1** is not longer maintained and a bit old, moving to **pnpm** we
get:
- ⚡ Significantly faster install times,
- 💾 Better disk space efficiency,
- 🛠️ Better community support and maintenance,
- 💆🏽‍♂️  Config swap very easy

##  🏗️ Changes

### Package Management Migration

- updated [corepack](https://github.com/nodejs/corepack) to use
[pnpm](https://pnpm.io/)
- Deleted `yarn.lock` and generated new `pnpm-lock.yaml`
- Updated `.gitignore`

### Documentation Updates

- `frontend/README.md`: 
  - added comprehensive tech stack overview with links
  - updated all commands to use pnpm
  - added corepack setup instructions
  - and included migration disclaimer for yarn users
- `backend/README.md`: 
  - Updated installation instructions to use pnpm with corepack
- `AGENTS.md`: 
  - Updated testing commands from yarn to pnpm

### CI/CD & Infrastructure

- **GitHub Workflows** : 
  - updated all jobs to use pnpm with corepack enable
  - cleaned FE Playwright test workflow to avoid Sentry noise
- **Dockerfile**:
- updated to use pnpm with corepack, changed lock file reference, and
updated cache mount path

###  📋 Checklist

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  
  **Test Plan:**
  > assuming you are on the `frontend` folder 
- [x] Clean installation works: `rm -rf node_modules && corepack enable
&& pnpm install`
  - [x] Development server starts correctly: `pnpm dev`
  - [x] Build process works: `pnpm build`
  - [x] Linting and formatting work: `pnpm lint` and `pnpm format`
  - [x] Type checking works: `pnpm type-check`
  - [x] Tests run successfully: `pnpm test`
  - [x] Storybook starts correctly: `pnpm storybook`
  - [x] Docker build succeeds with new pnpm configuration
  - [x] GitHub Actions workflow passes with pnpm commands

#### For configuration changes:
- [x] `.env.example` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)