feat: implement estimateFee method for stream operations

[GazzyLee]

Description

Before a user clicks 'Create Stream', the UI should show them the estimated network fee. This PR exposes estimateFee(operation: StreamOperation) on StreamsModule.

Changes

• src/types/index.ts: Added StreamOperation discriminated union (7 variants: create, withdraw, cancel, pause, resume, topUp, clawback) and FeeEstimate interface
• src/streams.ts: Added estimateFee(operation) method that builds the contract-call transaction, simulates it via Soroban RPC, and extracts the resource fee (minResourceFee), CPU instructions (cost.cpuInsns), and base fee (BASE_FEE) into a typed FeeEstimate response

Implementation

• Uses the existing _resolveCallerAddress(), _server(), buildContractCallTx(), getTokenDecimals(), and _resolveAddr() helpers
• Wraps simulation errors with RateLimitError.fromRpcError() for 429 handling
• Validates simulation result through SorobanRpc.Api.isSimulationError() — no as any property guessing
• Returns { totalFee, resourceFee, baseFee, instructions } in stroops

Testing

• tsc --noEmit: clean
• All existing tests pass (no test modifications)

Closes #353