fix(prefs): flush settings on every reboot/shutdown path, not just per-screen exit

Root cause of "settings don't survive a reboot": every screen that edits
NodePrefs (Settings, Bot, Trail, Locator, GPS sharing, etc.) only persisted
via its own savePrefsIfDirty(_dirty) call on its own Cancel/exit path.
UITask::shutdown() -- the one function behind every real power-off/reboot,
including the low-battery auto-shutdown that fires directly from
UITask::loop() regardless of which screen is on-screen -- never called
the_mesh.savePrefs() at all. A user who changed a setting and then hit any
of those paths without first explicitly backing out of the screen (e.g.
display auto-off mid-edit, then a low-battery auto-shutdown) silently lost
the change.

Also closes two direct board.reboot() bypasses that skipped shutdown()
entirely: the phone-app CMD_REBOOT command and the serial CLI "reboot"
command. The factory-reset reboot path is deliberately left alone -- it's
supposed to wipe everything.

Verified with a real regression test (not just "it compiles"): set a pref
in memory without flushing it, cross the real low_batt_mv threshold to
trigger the actual production low-battery-shutdown code path, then a real
page.reload(), then confirm behaviourally (does an incoming message wake
the screen or not) whether the setting survived. Temporarily reverted the
fix and re-ran to confirm the test actually fails without it (screen woke,
setting lost) before restoring it and confirming it passes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

examples/companion_radio/MyMesh.cpp

@@ -2383,6 +2383,7 @@ void MyMesh::handleCmdFrame(size_t len) {
     if (dirty_contacts_expiry) { // is there are pending dirty contacts write needed?
       saveContacts();
     }
+    savePrefs();  // flush any on-device setting change not yet persisted -- see UITask::shutdown()'s comment
     board.reboot();
   } else if (cmd_frame[0] == CMD_GET_BATT_AND_STORAGE) {
     uint8_t reply[11];
@@ -3191,6 +3192,7 @@ void MyMesh::checkCLIRescueCmd() {
       }
 
     } else if (strcmp(cli_command, "reboot") == 0) {
+      savePrefs();  // flush any on-device setting change not yet persisted -- see UITask::shutdown()'s comment
       board.reboot();  // doesn't return
     } else {
       Serial.println("  Error: unknown command");