Add Arbitrum (ARBETH) deployment support

[techiejd]

What

Adds deployment support for Arbitrum One (crypto key arbeth, component arbitrum), mirroring the polygon component:

• generator/constants.py — "arbeth": {"component": "arbitrum"}.
• generator/docker-components/arbitrum.yml — daemon service (image bitcart/bitcart-arbeth:stable, port 5012, datadir volume, backend links/depends_on).
• backend.yml / worker.yml — ARBETH_NETWORK env passthrough.
• setup.sh / helpers.sh — ARBETH_NETWORK help text, default, and env passthrough.
• dev-setup.sh — ARBETH_HOST=arbitrum.
• Demo page and README crypto lists.
• compose/arbeth.Dockerfile — generated via generate-templates.sh from bitcart's updated images.json (companion bitcart PR adds the bitcart-arbeth entry).

Note for maintainers: default server

ARBETH_SERVER defaults to the public https://arb1.arbitrum.io/rpc because seed-server.bitcart.ai does not currently serve Arbitrum. Happy to switch the default to a bitcart seed server if/when one is available. (Heads-up: public Arbitrum endpoints rate-limit aggressively — the Sepolia one 429'd during testing.)

Testing

• Generator test suite: 50/50 passing before and after.
• BITCART_CRYPTOS=btc,arbeth python -m generator produces a valid compose file with the arbitrum service, correct image, and ARBETH_NETWORK present in backend and worker environments.
• compose/arbeth.Dockerfile builds cleanly with the companion bitcart branch as build context (uv sync --frozen --group arbeth resolves in Alpine).
• The built image runs as a real deployment: started with ARBETH_NETWORK=testnet against Arbitrum Sepolia, getinfo on port 5012 returned connected: true, synchronized: true, live gas price, and the expected /home/electrum/.bitcart-arbeth/testnet datadir.

Dependencies

Depends on the companion bitcart PR (bitcart/bitcart#618: images.json entry, daemon) and bitcart-sdk PR (bitcart/bitcart-sdk#82: coin class).

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