GUI: Sort overview assets by receipt time

[Tabakkui]

Problem

The overview asset list sorts through an asset-name role passed as a column index, so it remains alphabetic rather than showing assets in the order they entered the wallet. Issue #875 requests date-based ordering.

Approach

• Track the earliest wallet transaction time represented by each asset's available outputs.
• Expose that timestamp as an asset model role.
• Configure the overview proxy to sort column 0 by that timestamp, newest first.

Test plan

• git diff --check
• Static regression assertions for timestamp capture, role exposure, and proxy sort configuration
• Full Qt build/test not run on the ARM64 Pi; requires the authorized PC build environment.

Fixes #875