v1.2: show real Android VPN RX/TX counters

[man612]

Summary

Adds real per-device traffic counters for connected Gnirehtet sessions without estimating usage.

Data source

• reads Android kernel network counters from /proc/net/dev through ADB
• considers only TUN interfaces
• if more than one TUN interface is visible, uses the one carrying the most traffic
• collects counters only for devices whose Gnirehtet client is actually active
• if the device/ROM does not expose usable TUN counters, Wirebound shows -- instead of fabricating values

UI

• adds a Traffic column with raw RX and TX byte totals formatted as B/KB/MB/GB
• exposes the underlying interface name in the cell tooltip

Tests

• covers /proc/net/dev parsing
• covers multiple TUN interfaces
• verifies the unavailable case remains undefined

These are kernel interface byte counters, not speed-test estimates.

Notes

This PR is stacked on #24 because both touch the device table. It will be retargeted to main after the multi-device control PR merges.