RFC: Add optional Bluetooth (bitchat-style) for private note transfer

Add an optional Bluetooth transport so users in close physical proximity can send private notes directly to each other, peer-to-peer, instead of routing them through the note transport layer. A standard P2ID/P2IDE note is only ~1–2 KB, so BLE transfer would be near-instant; private note transfer could be done with unauthenticated notes.

**Motivation**

- Improved privacy: private note state shared directly via local Bluetooth link
- Faster hand-off for in-person transfers (think point of sale terminal)

**Proposed behavior**

- Opt-in feature, disabled by default
- Discover nearby Miden wallet users over Bluetooth
- Establish a direct, encrypted link and transfer the note payload (bitchat-style)
- Fall back to the existing note transport layer when Bluetooth is unavailable or the peer is out of range

Look for existing Bluetooth integration or networking code in the wallet repository. Start by understanding the current note transport layer and how notes are structured. Research Bluetooth Low Energy (BLE) APIs for the target platform (likely mobile). The goal is to add a new transport option that discovers peers, establishes an encrypted link, and transfers note payloads, with a fallback to the existing layer.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.