Implement Real Transaction System for Wallet and RPC

[Steake]

Part of Epic: #69 - RC1: Core Transaction & State Infrastructure

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Goal: Enable creation, signing, submission, and processing of real transactions through GUI and RPC layer.

Requirements:

• Replace mock transaction formatting with construction of real Transaction structs in wallet GUI
• Use rpc_client.get_nonce for correct nonces
• Perform real cryptographic signing using wallet key
• Ensure raw transactions are serialized and sent over the wire
• Implement decoding, validation, and mempool inclusion on node RPC side
• Correctly fetch updated balances after transactions
• Priority: Blocking RC1 (Critical)

Relevant Files:

• crates/bitcell-wallet-gui/src/main.rs
• crates/bitcell-node/src/rpc.rs

Acceptance Criteria:

• GUI can create, sign and broadcast transactions
• Node receives, decodes, and processes transactions
• Balances update after transactions

See todo_now.md for audit steps.

[Steake]

[autopilot] @copilot please start work on this issue and open a PR that references #42 in the PR description.