Migrate monorepo to pnpm and add Expo React Native mobile client

[BODMAT]

Summary

Three related changes on this branch:

1. Migrate the monorepo from npm workspaces to pnpm. npm's hoisting is
   incompatible with Expo/Metro (it splits the package tree), which blocked
   adding a mobile app to the workspace.
2. Add an Expo (React Native) mobile client under apps/mobile, reusing
   the existing backend API.
3. Move CI/CD to pnpm so the pipeline installs and builds again.

What changed

pnpm migration (61f2ea2)

• pnpm-workspace.yaml (apps/*) replaces the workspaces field in package.json
• .npmrc with node-linker=hoisted — flat node_modules required by Metro/React Native
• packageManager: pnpm@11; package-lock.json removed in favor of pnpm-lock.yaml
• dev:mobile now runs pnpm --filter mobile run start
• Drift fixes surfaced by regenerating the lockfile:
  • add missing dotenv dependency to backend (phantom dep, exposed by pnpm strictness)
  • pin @types/express-serve-static-core to 5.1.0 (5.1.2 types query/params as string | string[])
  • bump frontend lib/target to ES2022 (for Array.at)
  • make the coin other schema field optional (zod 4.4 treats z.any() keys as required)
  • disable backend .d.ts emit to avoid TS2742 non-portable-type errors under pnpm

Mobile client (4c62fea)

• Expo SDK 54 app (expo-router, TypeScript) in apps/mobile
• Monorepo metro.config.js (resolves deps from local + workspace-root node_modules)
• No babel.config.js — SDK 54 auto-configures babel-preset-expo; a hand-written
  one breaks expo-router's EXPO_ROUTER_APP_ROOT inlining

CI/CD to pnpm (8b05bae)

• ci.yml and deploy.yml: add pnpm/action-setup, switch setup-node cache to pnpm
• npm ci → pnpm install --frozen-lockfile; npm run … → pnpm run …
• Bump node-version 20 → 22 (pnpm 11 requires Node ≥ 22.13; Node 20 is also deprecated on runners)
• Fixes the CI failures "Dependencies lock file is not found" and the node:sqlite crash on Node 20

Verification

• pnpm install --frozen-lockfile + native builds (bcrypt, esbuild, prisma) OK
• pnpm build (turbo) — backend + frontend green
• expo export --platform android — mobile bundles successfully
• dev servers (pnpm dev) unaffected

Notes

• pnpm requires approving dependency build scripts — handled via allowBuilds in pnpm-workspace.yaml
• ssh2's optional native binding may skip compilation on machines without build tools; it falls back to pure JS and is only used by integration tests