Node 24, and stop pointing at the mobile app that left the repository (#53)

mobile/ was moved out in April (18d41743), but the root package.json still
ran it: `npm run build`, `lint`, `test` and `dev` all called into a missing
directory and failed. The scripts now cover what is here (web, sign,
backend), concurrently is gone with the only command that needed it, and the
root lockfile shrinks to nothing.

The README described the mobile app and not the signing portal; it now lists
the real layout (web, sign, backend, shared, ops).

Node 20 is past its end of life. CI and both images move to Node 24; the
backend's sqlite3 native module builds and its tests pass on 24.

README.md

@@ -7,9 +7,10 @@ Monorepo for Pezkuwi blockchain frontend applications.
 ```
 pwap/
 ├── web/                    # Main web application
-├── mobile/                 # Mobile application (React Native + Expo)
-├── backend/                # Backend API services
+├── sign/                   # Multisig signing portal
+├── backend/                # Indexer service
 ├── shared/                 # Shared code and utilities
+├── ops/                    # CI deploy gate checks and host deploy scripts
 └── package.json            # Root package with build scripts
 ```
 
@@ -52,29 +53,18 @@ npm install
 npm run dev
 ```
 
-### 2. `mobile/` - Mobile Application
+### 2. `sign/` - Multisig Signing Portal
 
-**Status:** 🚧 In Development
-
-React Native Expo app for iOS and Android.
-
-**Features:**
-- Welcome screen with language selection
-- Multi-language support (6 languages with RTL)
-- Authentication (Sign In/Up)
-- Main dashboard navigation (5-tab bottom nav)
-- Wallet integration with @pezkuwi/api
-- Live blockchain data (HEZ, PEZ, USDT)
-- Send/receive transactions
-- Biometric authentication
+A deliberately small app with one job: approve and sign pending multisig
+operations. It compiles `shared/lib` and is typechecked and built in CI.
 
 ```bash
-cd mobile
+cd sign
 npm install
-npm start
+npm run build
 ```
 
-### 3. `backend/` - Backend Services
+### 3. `backend/` - Indexer Service
 
 API services for the applications.
 
@@ -101,7 +91,7 @@ shared/
 ## Quick Start
 
 ### Prerequisites
-- Node.js 18+
+- Node.js 24
 - npm
 
 ### Installation
@@ -116,7 +106,7 @@ npm install
 
 # Or install individually
 npm run install:web
-npm run install:mobile
+npm run install:sign
 npm run install:backend
 ```
 
@@ -128,18 +118,12 @@ npm run build
 
 This builds:
 1. `web` - Vite production build
-2. `pezkuwi-sdk-ui` - Full SDK UI build (separate repo)
-3. `mobile` - Expo web export
+2. `sign` - typecheck and Vite production build
 
 ### Development
 
 ```bash
-# Run web and mobile in parallel
-npm run dev
-
-# Or run individually
 npm run dev:web
-npm run dev:mobile
 ```
 
 ## Multi-Language Support