feat: surface nearby LAN devices in Connect screen via Syncthing local discovery

[gitcoder89431]

Summary

Syncthing already discovers devices on the local network via multicast UDP (port 21027) and exposes them at /rest/system/discovery. meshd never reads this endpoint, so users have to manually exchange device IDs even when both machines are on the same WiFi.

What Syncthing already knows

/rest/system/discovery returns every device Syncthing has seen locally, with their LAN addresses:

{
  "O2GLNK3-...": {
    "addresses": [
      "tcp://192.168.1.153:22000",
      "quic://192.168.1.153:22000",
      ...
    ]
  }
}

This includes unpaired devices — Syncthing hears their multicast announcements but meshd never surfaces them.

Proposed UX

In the Connect screen, add a Nearby section above Pending:

Connect //////////////////////////////////////////////////////////////
This device
  leaf-mac  A7OSXBE-...

Nearby — on your local network
  EN3JEAY   192.168.1.153    [a] pair

Approved
  mini-host  ● Online

Pending
  (none)

• Poll /rest/system/discovery and cross-reference against already-paired device IDs
• Devices with a LAN address (192.168.x.x, 10.x.x.x, 172.16-31.x.x, fe80::) that aren't already paired show up as Nearby
• Press a to pair — calls AcceptDevice same as accepting a pending device
• No ID exchange needed when both machines are on the same network

Why this matters

This is the same zero-config discovery LocalSend uses (mDNS/multicast, TTL-limited to the local subnet). Syncthing is already doing the hard part — meshd just needs to expose it. Pairing two machines on the same LAN should be: open meshd on both, press a, done.