v1.7.8 — Speed Test: iperf3 LAN tab

Speed Test — new LAN (iperf3) tab

Alongside the existing internet speed test, a new LAN tab runs client-only iperf3 tests. Suggested by issue #6.

• Choose a server from a bundled public list (26 countries) or save your own custom server (e.g. an internal org iperf3 server)
• TCP or UDP, IPv4/IPv6/auto, and reverse (download) direction
• Jitter and packet loss shown for UDP tests
• Client only for now — no server mode

New optional dependency: iperf3 — install it to enable the tab; without it the tab shows a disabled banner instead of failing silently.

See full changelog in README.md.