Merge PR #37: lock screen PIN (3urobeat), adapted

The contributor's feature -- a PIN asked before the screen unlocks, on
boot, and when the magnet cover opens, stored as a salted SHA-256 -- on
top of this branch, with:

- ui-core/ScreenLock.h: set / check / isSet and the tries limit, shared by
  L1 and L2. isSet looks at every byte (the PR tested only the first, so one
  PIN in 256 silently disabled the lock). 5 misses pause entry for 30 s.
- NodePrefs: the fields at the tail after quiet hours, sentinel 0xC0DE0031,
  sizeof 2880; an older file's bytes there are cleared. A sentinel bump no
  longer resets display_brightness_pct from a file that already has it.
- DataStore: no AGPL header (the project is MIT), no whitespace churn.
- Keyboard: the number pad is a widget flag (pin_mode) with a prompt in the
  preview ("New PIN", "Again", "PIN"), not keyboard_type = 2 written into the
  prefs and restored after.
- L1 Settings: "Lock PIN" right after the schema's "Lock screen" row, typed
  twice, at least 4 characters, saved at once (a power-off right after
  setting it kept no PIN). Home is told about a boot-time lock once it
  exists (also fixes booting with the cover closed).
- L2: the screen PIN moves from plain text in NVS to the same hash in
  NodePrefs; the old one is migrated on first boot and removed. The pad
  unlocks as soon as the hash matches from 4 digits on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-lvgl/UITask.cpp

@@ -898,8 +898,15 @@ void UITask::begin(DisplayDriver* display_drv, SensorManager* sensors, NodePrefs
   buildStatusBar();
   applyDisplayPrefs();   // a slider percentage overrides the level main.cpp set
   showHome();
-  lvport::loadPin(_pin, sizeof(_pin));
-  if (_pin[0]) lockScreen();   // a reboot doesn't get round the PIN
+  // Before the PIN moved into NodePrefs it was kept in NVS in plain text.
+  char old_pin[9];
+  lvport::takeOldPin(old_pin, sizeof(old_pin));
+  if (old_pin[0] && _prefs && !pinSet()) {
+    screenlock::set(*_prefs, old_pin, the_mesh.getRNG());
+    the_mesh.savePrefs();
+  }
+  memset(old_pin, 0, sizeof(old_pin));
+  if (pinSet()) lockScreen();   // a reboot doesn't get round the PIN
   showSplash();                // over both; fades out by itself
 }
 
@@ -1175,7 +1182,7 @@ void UITask::sleep() {
   if (_display) _display->turnOff();
   prefsSaveSoon(0);   // nothing to stall now
   lvport::powerSave(true, _tap_wake);
-  if ((_prefs && _prefs->auto_lock) || _pin[0]) lockScreen();   // Lock screen, or a screen PIN
+  if ((_prefs && _prefs->auto_lock) || pinSet()) lockScreen();   // Lock screen, or a screen PIN
 }
 
 void UITask::wake() {
@@ -3212,7 +3219,7 @@ void UITask::buildSchemaSettings() {
       lv_obj_add_event_cb(tw, onTapWake, LV_EVENT_VALUE_CHANGED, NULL);
       g = group(body, "LOCK");
       schemaRow(g, SETTING(auto_lock));
-      listRow(g, "Screen PIN", _pin[0] ? "On  -  asked when the screen wakes" : "Off", onPinSetup, NULL);
+      listRow(g, "Screen PIN", pinSet() ? "On  -  asked when the screen wakes" : "Off", onPinSetup, NULL);
       accentRow(group(body, "LOOK"));   // DeviceScreen.h
       return;
     }