feat: introduce Amount type to standardize msat/sat units across the protocol

[portal-wheatley]

Problem

Across the codebase, amounts are passed as raw integers (u64, i64, long) with no enforced unit. This leads to ambiguity and potential bugs when different parts of the stack use different conventions — e.g. pay_invoice returning fees in sats vs the protocol defaulting to msat.

Related: the pay_invoice trait was recently updated to return fees in sats (#149), which surfaced this inconsistency.

Proposal

Introduce an Amount type in portal (core crate) that makes the unit explicit:

pub enum Amount {
    Millisats(u64),
    Sats(u64),
}

impl Amount {
    pub fn to_msat(&self) -> u64 { ... }
    pub fn to_sat(&self) -> u64 { ... }
}

All protocol-level structs (payment requests, wallet trait, responses) should use Amount instead of bare integers, eliminating implicit unit assumptions.

Benefits

• No more silent msat/sat mismatches
• Easier to audit and refactor payment flows
• Cleaner FFI/bindings (the type carries its unit)

Scope

• portal core crate: payment request/response types
• portal-wallet trait (pay_invoice, get_wallet_info, etc.)
• portal-rest wire format (serialization stays as msat for protocol compatibility)
• Bindings (UniFFI): expose Amount as a proper enum