feat: improve shipping zone UX

- Remove asterisks from UK/US country names (#153)
- Remove unclear "Flat rate" option (#153)
- Alphabetize country list, move China to correct position (#153)
- Rename "Unit" to "Currency" (#154)
- Rename cost label to "Default shipping cost" (#154)
- Add currency suffix to cost field (#154)
- Add hint about per-product shipping costs (#154)
- Add validation for whole number sats (#155)
- Disable submit button when sats validation fails (#155)
- Show error message for invalid sats values (#155)

Closes #153, #154, #155

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,104 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Nostr Market is an LNbits extension implementing NIP-15 (decentralized marketplace protocol) on Nostr. It enables merchants to create webshops (stalls) and sell products with Lightning Network payments, featuring encrypted customer-merchant communication via NIP-04.
+
+**Prerequisites:** Requires the LNbits [nostrclient](https://github.com/lnbits/nostrclient) extension to be installed and configured.
+
+## Common Commands
+
+All commands are in the Makefile:
+
+```bash
+make format      # Run prettier, black, and ruff formatters
+make check       # Run mypy, pyright, black check, ruff check, prettier check
+make test        # Run pytest with debug mode
+make all         # Run format and check
+```
+
+Individual tools:
+
+```bash
+make black       # Format Python files
+make ruff        # Check and fix Python linting
+make mypy        # Static type checking
+make pyright     # Python static type checker
+make prettier    # Format JS/HTML/CSS files
+```
+
+## Local Development Setup
+
+To run checks locally, install dependencies:
+
+```bash
+# Install Python autotools dependencies (needed for secp256k1)
+sudo apt-get install -y automake autoconf libtool
+
+# Install Python dependencies
+uv sync
+
+# Install Node dependencies (for prettier)
+npm install
+
+# Run all checks
+make check
+```
+
+## Architecture
+
+### Core Layers
+
+1. **API Layer** (`views_api.py`) - REST endpoints for merchants, stalls, products, zones, orders, direct messages
+2. **Business Logic** (`services.py`) - Order processing, Nostr event signing/publishing, message routing, invoice handling
+3. **Data Layer** (`crud.py`) - Async SQLite operations via LNbits db module
+4. **Models** (`models.py`) - Pydantic models for all entities
+
+### Nostr Integration (`nostr/`)
+
+- `nostr_client.py` - WebSocket client connecting to nostrclient extension for relay communication
+- `event.py` - Nostr event model, serialization, ID computation (SHA256), Schnorr signatures
+
+### Background Tasks (`__init__.py`, `tasks.py`)
+
+Three permanent async tasks:
+
+- `wait_for_paid_invoices()` - Lightning payment listener
+- `wait_for_nostr_events()` - Incoming Nostr message processor
+- `_subscribe_to_nostr_client()` - WebSocket connection manager
+
+### Frontend (`static/`, `templates/`)
+
+- Merchant dashboard: `templates/nostrmarket/index.html`
+- Customer marketplace: `templates/nostrmarket/market.html` with Vue.js/Quasar in `static/market/`
+- Use Quasar UI components when possible: https://quasar.dev/components
+
+### Key Data Models
+
+- **Merchant** - Shop owner with Nostr keypair, handles event signing and DM encryption
+- **Stall** - Individual shop with products and shipping zones (kind 30017)
+- **Product** - Items for sale with categories, images, quantity (kind 30018)
+- **Zone** - Shipping configuration by region
+- **Order** - Customer purchases with Lightning invoice tracking
+- **DirectMessage** - Encrypted chat (NIP-04)
+- **Customer** - Buyer profile with Nostr pubkey
+
+### Key Patterns
+
+- **Nostrable Interface** - Base class for models convertible to Nostr events (`to_nostr_event()`, `to_nostr_delete_event()`)
+- **Parameterized Replaceable Events** - Stalls (kind 30017) and Products (kind 30018) per NIP-33
+- **AES-256 Encryption** - Customer-merchant DMs use shared secret from ECDH
+- **JSON Meta Fields** - Complex data (zones, items, config) stored as JSON in database
+
+### Cryptography (`helpers.py`)
+
+- Schnorr signatures for Nostr events
+- NIP-04 encryption/decryption
+- Key derivation and bech32 encoding (npub/nsec)
+
+## Workflow
+
+- Always check GitHub Actions after pushing to verify CI passes
+- Run `make check` locally before pushing to catch issues early