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