[UI][Proposals] Verify account existence when create a new proposal

When we create a new proposal, to transfer some tez to an address (implicit account or smart contract) we need to check if this account exists.

The issue names no file, test, or entry point. Start by tracing the TypeScript proposal-creation flow and inspect how recipient addresses are handled; done means the flow verifies whether an implicit account or smart contract exists before creating a proposal, with the expected behavior covered by the relevant tests.

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