fix: restore fixed position into localPosition on boot

[cvaldess]

Problem

Fixed-position nodes without a GPS stop broadcasting their position — and stop publishing MQTT map reports — after a reboot. The persisted fixed position is still in the NodeDB (it shows up in --info), but the node never sends a position again until a position is manually re-set during the same session.

Root cause

On boot localPosition (RAM) starts empty and nothing repopulates it from the persisted fixed position. NodeDB::hasValidPosition() for the local node only inspects localPosition, not the stored node position:

if (n->num == getNodeNum()) {
    return localPosition.latitude_i != 0 || localPosition.longitude_i != 0;
}

So PositionModule::runOnce() never reaches sendOurPosition() (its hasValidPosition(node) gate is false), the lazy localPosition backfill inside getPositionPacket() never runs, and localPosition stays (0, 0) indefinitely. The deadlock is normally broken only by a live GPS fix or by an admin position set in the same session — neither of which happens on a headless fixed-position node after a reboot.

Fix

During NodeDB construction, when config.position.fixed_position is set, restore the persisted node position into localPosition. Position broadcasts and MQTT map reports then resume automatically after a reboot, with no manual intervention.

Validation

Reproduced and verified on RP2350 + W5500 boards (fixed position, no GPS):

• Before: MQTT Map report enabled, but no position available after every reboot, and no MAP_REPORT_APP reaching the broker.
• After: localPosition is restored at boot (Restored fixed position to localPosition: ...), position broadcasts resume, and the map report publishes to msh/<region>/2/map/ on each reboot.