AppForge: migrate off Supabase to external Postgres + Better Auth + local storage

- New Express + TypeScript backend (server/) with pg, Better Auth, local file storage
- De-Supabased Postgres schema (server/db) and TS reimplementations of DB functions
- Frontend data layer rewired to REST (rest-client + backend-client compat shim)
- Removed all Supabase references (code, config, deps, docs)
- New brand assets: gradient favicon/app icons + dark/white wordmark logos

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

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+# ============================================
+# AppForge - Environment Variables
+# Stack: External Postgres + Better Auth + Local Storage + Express backend
+# Copy this file to .env and fill in your values. Never commit .env!
+# ============================================
+
+# ============================================
+# Database (Required)
+# ============================================
+# Postgres connection string for your external database
+DATABASE_URL=postgres://user:password@host:5432/postgres
+# Set to true to allow self-signed / non-verified TLS on the DB connection
+PGSSL_NO_VERIFY=true
+
+# ============================================
+# Backend Server (Required)
+# ============================================
+# Port the Express API server listens on
+PORT=8787
+
+# ============================================
+# Better Auth (Required)
+# ============================================
+# Generate a strong random secret for production (e.g. `openssl rand -hex 32`)
+BETTER_AUTH_SECRET=change-me-to-a-long-random-string
+# Public origin the browser uses (dev: the Vite app origin)
+BETTER_AUTH_URL=http://localhost:8080
+# Optional Google OAuth (leave blank to disable Google sign-in)
+GOOGLE_CLIENT_ID=
+GOOGLE_CLIENT_SECRET=
+
+# ============================================
+# Local File Storage
+# ============================================
+# Directory where uploaded files are stored (relative to project root or absolute)
+STORAGE_DIR=./server/storage
+# Public URL prefix used to serve stored files
+STORAGE_PUBLIC_PREFIX=/storage
+
+# ============================================
+# Self-Hosting / App Defaults
+# ============================================
+# Email that is auto-assigned the admin role on first signup (optional)
+INITIAL_ADMIN_EMAIL=admin@example.com
+NODE_ENV=development
+
+# ============================================
+# Frontend (Vite)
+# ============================================
+# Base URL the frontend uses to reach the backend. Empty = same origin (Vite proxy).
+VITE_API_URL=
+# Demo mode: true | false | empty (defer to database setting)
+VITE_DEMO_MODE=false
+
+# ============================================
+# Optional Integrations (server-side; fill when needed)
+# ============================================
+# STRIPE_SECRET_KEY=
+# STRIPE_WEBHOOK_SECRET=
+# PAYPAL_CLIENT_ID=
+# PAYPAL_CLIENT_SECRET=
+# COINBASE_API_KEY=
+# COINBASE_WEBHOOK_SECRET=
+# CODEMAGIC_API_TOKEN=
+# CODEMAGIC_APP_ID=
+# RESEND_API_KEY=
+# OPENAI_API_KEY=
+# APPETIZE_API_KEY=