chore: tron snap code quality improvements

[ulissesferreira]

Summary

• Remove entities/index.ts barrel file and use direct module imports for better clarity and tree-shaking
• Add limit parameter to TrongridApiClient.getTransactionInfoByAddress to support lightweight queries
• Add checkAddressActivity method to TransactionsService for efficient account discovery (queries with limit=1 instead of downloading full transaction history)
• Use checkAddressActivity in discoverAccounts instead of fetchNewTransactionsForAccount
• Fix cursor handling in listAccountTransactions — return empty results when cursor is not found instead of starting from index -1
• Move account sync earlier in trackTransaction so it still runs even if the sender account lookup fails
• Remove unused params: { accountIds } from setSelectedAccounts background event call
• Narrow findByIds return type from TronKeyringAccount[] | null to TronKeyringAccount[] (removing impossible null)
• Narrow synchronize* method parameters from KeyringAccount to TronKeyringAccount
• Remove unnecessary type cast account as TronKeyringAccount in TransactionsMapper call

Test plan

• Verify all existing tests pass
• Verify checkAddressActivity tests cover true/false/error cases
• Verify discoverAccounts tests use the new checkAddressActivity mock
• Verify no functional regressions in account discovery, transaction tracking, or pagination

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