fix(cli): persist selected network and use as default across invocations (closes #10)

[ghzhost]

Summary

Persists the selected network (via Commands::Switch or disk storage) so subsequent commands default to the previously chosen network rather than silently resetting to Mainnet on every invocation.

Changes

• Utils (src/utils/mod.rs):
  • Added save_current_network / save_network and get_current_network to persist/load network preference from ~/.txio/current_network mirroring the existing current_chain mechanism.
• Parser (src/cli/parser.rs):
  • Made the global --network flag optional (Option<Network>) so CLI invocations can distinguish between explicitly passing a network and omitting it to fall back to the persisted setting.
  • Implemented FromStr for Network.
  • Updated Commands::Switch to accept optional chain: Option<String> and optional --network flag (network: Option<Network>).
• Handlers (src/cli/handlers.rs):
  • Added CommandHandler::resolve_network(explicit_network: Option<Network>) -> Network implementing the precedence:
    1. Explicit --network flag (if provided).
    2. Persisted network in ~/.txio/current_network (if present and valid).
    3. Default fallback to Network::Mainnet.
  • Updated Commands::Switch to persist network when --network is supplied, along with chain switching.
  • Updated Commands::Status and all chain command adapters (Sui, Ethereum, Solana, Aptos, Soroban) to use the effective network.
• Unit Tests:
  • Added test coverage in src/utils/mod.rs for network persistence round-trip.
  • Added test coverage in src/cli/handlers.rs for resolve_network precedence (first run default, persisted setting, and explicit override).

Closes #10

[Kingvic300]

@ghzhost kindly resolve conflicts