feat: publish device on the LAN via mDNS / Bonjour

[PAzter1101]

Summary

Adds mDNS / Bonjour publishing so the device is reachable as <hostname>.local from any machine on the same LAN, without needing a static IP or a router-side DHCP reservation. The Local TCP Server (when enabled) is also advertised as a _reticulum._tcp service for auto-discovery by Reticulum-aware tooling.

In configuration-portal AP mode the captive portal is reachable as rtnode.local, removing the need to remember 10.0.0.1.

Motivation

Right now the only way to point an external rnsd (or Sideband) at a transport node's Local TCP Server is to look the address up in the router admin and either set up a DHCP reservation or hard-code the lease. Both break the moment the lease changes hands or the user moves to a different network. mDNS solves it with a stable, human-readable name.

What's in the patch

Configuration portal

New Local Network Name (mDNS) section:

• mDNS — Enable / Disable (default Enabled). When disabled, the device suppresses all multicast announcements and is only reachable by IP.
• Hostname — optional custom hostname. Empty falls back to rtnode<XXXX> where XXXX is the last 4 hex chars of the device MAC. Input is normalised to lowercase + [a-z0-9-] and stripped of leading/trailing hyphens before save.

OLED display

When mDNS is enabled, the right-panel lower area shows three rows: hostname, IP, port. The previous bottom separator line is dropped to free the row.

When mDNS is disabled, the original two-row layout (IP + port + separator) is preserved unchanged.

EEPROM additions

Address	Size	Purpose
0x151	1 byte	mDNS enable flag
0x152	33 bytes	Custom hostname (null-terminated)

Both default to "enabled / auto-generated name" when the bytes are unprogrammed (0xFF), so existing installations keep working without any factory reset.

Implementation

mDNS itself is a thin header-only wrapper around ESPmDNS in MdnsService.h. It is started lazily from the main loop once WiFi finishes its asynchronous association, and torn down before switching the radio into config-portal AP mode so the ESPmDNS state never survives a WiFi mode change.

Build fix (separate commit)

platformio.ini had MsgPack@^0.4.2 which is now ambiguous between hideakitai/MsgPack and mbed-yihui/msgpack and stalls the Library Manager. Pinned to hideakitai/MsgPack@^0.4.2 to keep the same version range while removing the prompt. This is its own commit so you can pick it up or skip it.

Backwards compatibility

• New EEPROM bytes default to enabled-with-auto-name on uninitialised (0xFF) reads, so users who flash this build over an existing install get mDNS automatically without losing any saved settings.
• Users who don't want mDNS can disable it in the portal — the OLED layout reverts to the original look in that case.
• No changes to the LoRa, TCP backbone, or Local TCP Server code paths.

Testing

Tested on a Heltec V4 (V4.3 revision):

• mDNS resolves rtnode<XXXX>.local from a Linux host (avahi-resolve, getent hosts)
• _reticulum._tcp service appears in avahi-browse -art _reticulum._tcp with the correct port and address
• rnsd connects to the Local TCP Server using the .local name as target_host
• Custom hostname round-trips through the portal save/load
• Disabling mDNS cleanly stops the service and reverts the OLED layout
• Config-portal AP mode reachable as rtnode.local

Not tested on Heltec V3 — the implementation only touches the WiFi/Boundary path and uses standard ESPmDNS APIs supported on all ESP32-S3 board variants in this repo, so I expect it to work, but a V3 owner would need to confirm.

Notes

• Routers with strict AP/client isolation or aggressive IGMP snooping may swallow the multicast and prevent resolution. This is an environmental thing, not a firmware bug — the same setting also blocks any other mDNS device on the network.
• mDNS adds roughly ~2 KB of flash and a few hundred bytes of heap. Did not observe any memory regressions on the V4 (16 MB / 2 MB PSRAM).

[jrl290]

I'll check this out. Probably closer to the weekend

Just fyi, the IP address is printed on the device screen