feat: support multiple RPC providers per chain with fallback

Problem

Each chain has a single rpc: string endpoint. If that endpoint is unreachable, the CLI fails with no automatic recovery. This is especially problematic for public RPC endpoints which can have downtime.

Proposal

Allow configuring multiple RPC endpoints per chain with automatic fallback.

Type change (src/config/types.ts)

export interface ChainConfig {
  rpc: string | string[];  // single URL or ordered list (first = primary)
  lightClient?: boolean;
}

Fallback behavior

• Try endpoints in order; on connection failure or timeout, try the next
• Log which endpoint was selected
• The --rpc CLI flag overrides all configured endpoints with the given URL

CLI changes

• dot chain add <name> --rpc wss://a --rpc wss://b — add chain with multiple RPCs
• Consider dot chain rpc add/remove/list <name> subcommands for managing endpoints on existing chains

Example built-in config with multiple providers

polkadot: {
  rpc: [
    "wss://rpc.polkadot.io",
    "wss://polkadot-rpc.dwellir.com",
    "wss://rpc.ibp.network/polkadot",
  ],
}

Files to modify

• src/config/types.ts — change rpc type to string | string[]
• src/core/client.ts — implement fallback logic in createChainClient
• src/commands/chain.ts — support multiple --rpc flags
• src/config/store.ts — ensure config read/write handles arrays

Considerations

• Backward compatible: existing configs with rpc: "wss://..." still work
• Investigate if polkadot-api's getWsProvider already supports multiple URLs before building custom fallback
• Keep it simple: only fail over on connection error, no proactive health checks

Start with src/config/types.ts and investigate whether polkadot-api's getWsProvider already accepts multiple URLs. Trace createChainClient in src/core/client.ts, then review src/commands/chain.ts and src/config/store.ts for CLI parsing and config persistence. Done means ordered fallback works for connection failures and timeouts, logging identifies the selected endpoint, and a single --rpc override remains supported.