chore: migrate frontend Dockerfile from yarn to pnpm

[Shitanshukumar607]

Running docker compose up -d --build failed during the frontend build step:
target frontend: failed to solve: failed to compute cache key: ... "/yarn.lock": not found

This occurred because frontend/Dockerfile was referencing yarn.lock and yarn install, whereas the repository has migrated to pnpm (pnpm-lock.yaml).

Fix

• Enabled Corepack in frontend/Dockerfile for native pnpm support.
• Replaced yarn.lock with pnpm-lock.yaml in the COPY instruction.
• Updated yarn install and yarn dev to pnpm install and pnpm dev --host 0.0.0.0.