Allow setting RTC clock backwards and fix elapsed-time underflow

Remove the "clock cannot go backwards" restriction from all set-time
paths (CLI `time`, `clock sync`, companion radio CMD_SET_DEVICE_TIME,
and simple_secure_chat). The ESP32-S3 RTC drifts 5-10% during deep
sleep, making backwards correction necessary after even a few days.

Add safeElapsedSecs() helper in ArduinoHelpers.h that clamps elapsed
time to 0 when a stored timestamp appears to be in the future after a
clock correction. Applied to:
- Neighbor "heard X ago" displays in simple_repeater
- UI time displays in companion_radio
- TimeSeriesData calculations in simple_sensor

Switch BaseChatMesh connection expiry from RTC timestamps to monotonic
millis(), making it immune to RTC adjustments from GPS, NTP, or manual
sync. Rename last_activity to last_activity_ms to reflect the change.

examples/companion_radio/MyMesh.cpp

@@ -1193,13 +1193,8 @@ void MyMesh::handleCmdFrame(size_t len) {
   } else if (cmd_frame[0] == CMD_SET_DEVICE_TIME && len >= 5) {
     uint32_t secs;
     memcpy(&secs, &cmd_frame[1], 4);
-    uint32_t curr = getRTCClock()->getCurrentTime();
-    if (secs >= curr) {
-      getRTCClock()->setCurrentTime(secs);
-      writeOKFrame();
-    } else {
-      writeErrFrame(ERR_CODE_ILLEGAL_ARG);
-    }
+    getRTCClock()->setCurrentTime(secs);
+    writeOKFrame();
   } else if (cmd_frame[0] == CMD_SEND_SELF_ADVERT) {
     mesh::Packet* pkt;
     if (_prefs.advert_loc_policy == ADVERT_LOC_NONE) {

examples/companion_radio/ui-new/UITask.cpp

@@ -233,7 +233,7 @@ public:
       for (int i = 0; i < UI_RECENT_LIST_SIZE; i++, y += 11) {
         auto a = &recent[i];
         if (a->name[0] == 0) continue;  // empty slot
-        int secs = _rtc->getCurrentTime() - a->recv_timestamp;
+        uint32_t secs = safeElapsedSecs(_rtc->getCurrentTime(), a->recv_timestamp);
         if (secs < 60) {
           sprintf(tmp, "%ds", secs);
         } else if (secs < 60*60) {
@@ -496,7 +496,7 @@ public:
 
     auto p = &unread[head];
 
-    int secs = _rtc->getCurrentTime() - p->timestamp;
+    uint32_t secs = safeElapsedSecs(_rtc->getCurrentTime(), p->timestamp);
     if (secs < 60) {
       sprintf(tmp, "%ds", secs);
     } else if (secs < 60*60) {

examples/simple_repeater/MyMesh.cpp

@@ -355,7 +355,7 @@ int MyMesh::handleRequest(ClientInfo *sender, uint32_t sender_timestamp, uint8_t
 #if MAX_NEIGHBOURS
         // add next neighbour to results
         auto neighbour = sorted_neighbours[index + offset];
-        uint32_t heard_seconds_ago = getRTCClock()->getCurrentTime() - neighbour->heard_timestamp;
+        uint32_t heard_seconds_ago = safeElapsedSecs(getRTCClock()->getCurrentTime(), neighbour->heard_timestamp);
         memcpy(&results_buffer[results_offset], neighbour->id.pub_key, pubkey_prefix_length); results_offset += pubkey_prefix_length;
         memcpy(&results_buffer[results_offset], &heard_seconds_ago, 4); results_offset += 4;
         memcpy(&results_buffer[results_offset], &neighbour->snr, 1); results_offset += 1;
@@ -1042,7 +1042,7 @@ void MyMesh::formatNeighborsReply(char *reply) {
     mesh::Utils::toHex(hex, neighbour->id.pub_key, 4);
 
     // add next neighbour
-    uint32_t secs_ago = getRTCClock()->getCurrentTime() - neighbour->heard_timestamp;
+    uint32_t secs_ago = safeElapsedSecs(getRTCClock()->getCurrentTime(), neighbour->heard_timestamp);
     sprintf(dp, "%s:%d:%d", hex, secs_ago, neighbour->snr);
     while (*dp)
       dp++; // find end of string

examples/simple_secure_chat/main.cpp

@@ -158,13 +158,8 @@ class MyMesh : public BaseChatMesh, ContactVisitor {
   }
 
   void setClock(uint32_t timestamp) {
-    uint32_t curr = getRTCClock()->getCurrentTime();
-    if (timestamp > curr) {
-      getRTCClock()->setCurrentTime(timestamp);
-      Serial.println("   (OK - clock set!)");
-    } else {
-      Serial.println("   (ERR: clock cannot go backwards)");
-    }
+    getRTCClock()->setCurrentTime(timestamp);
+    Serial.println("   (OK - clock set!)");
   }
 
   void importCard(const char* command) {

examples/simple_sensor/TimeSeriesData.cpp

@@ -1,4 +1,5 @@
 #include "TimeSeriesData.h"
+#include <helpers/ArduinoHelpers.h>
 
 void TimeSeriesData::recordData(mesh::RTCClock* clock, float value) {
   uint32_t now = clock->getCurrentTime();
@@ -12,7 +13,7 @@ void TimeSeriesData::recordData(mesh::RTCClock* clock, float value) {
 
 void TimeSeriesData::calcMinMaxAvg(mesh::RTCClock* clock, uint32_t start_secs_ago, uint32_t end_secs_ago, MinMaxAvg* dest, uint8_t channel, uint8_t lpp_type) const {
   int i = next, n = num_slots;
-  uint32_t ago = clock->getCurrentTime() - last_timestamp;
+  uint32_t ago = safeElapsedSecs(clock->getCurrentTime(), last_timestamp);
   int num_values = 0;
   float total = 0.0f;