feat(tusdt-cli): add dry-run and Ledger hardware wallet support

[Benup211]

Add --dry-run for gas/fee preview without submitting, and --ledger
for signing with Ledger Nano S/X via USB HID (Polkadot generic app).

Dry-run:

• DryRunResult dataclass in client.py
• _dry_run() performs read-only runtime simulation via ContractsApi::call
• _estimate_fee() queries TransactionPaymentApi_query_info (best-effort)
• dry_run flag on TUSDTClient, branch in _exec()
• Output.dry_run_result() renders gas/fee/status/return-value panel
• --dry-run global flag, sys.exit(0) after preview (no success msg)

Ledger:

• ledger.py: LedgerSigner (USB HID + APDU), LedgerKeypair(Keypair)
  subclass, find_ledger_device(), BIP44 derivation
• m/44'/354'/ACCOUNT'/0'/INDEX' (SLIP-44 coin type 354 = Polkadot)
• --ledger shorthand for --signer-backend ledger (btcli pattern)
• --ledger-account/--ledger-index for BIP44 path selection
• CLIContext.uses_ledger() dispatches in submit(), skips get_signer_keypair
• hid optional via pip install tusdt-cli[ledger]
• LEDGER_ERROR code + remediation in errors.py

Fixes:

• _gas_to_int moved before class (was closing TUSDTClient prematurely)
• _exec() and all contract methods typed as dict[str, Any] | DryRunResult
• ScaleBytes handled in LedgerKeypair.sign()
• hid installed as dev dependency for type checking