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.h

@@ -16,6 +16,7 @@
 #include "../AbstractUITask.h"
 #include "../NodePrefs.h"
 #include "../ui-core/UiCoreHost.h"
+#include "../ui-core/ScreenLock.h"
 
 class UiCore;
 class NearbyModel;
@@ -458,13 +459,12 @@ private:
   uint16_t _batt_mv = 0;         // smoothed, read every 8 s: the status bar and the low-battery shutdown
   uint32_t _next_batt_ms = 0;
   lv_obj_t* _prune_lbl = nullptr;
-  // Screen PIN (DeviceScreen.h); stored in NVS (lvport::loadPin)
-  char      _pin[9] = "";          // "" = no PIN
+  // Screen PIN (DeviceScreen.h): a salted hash in NodePrefs (ui-core/ScreenLock.h)
+  bool pinSet() const { return _prefs && screenlock::isSet(*_prefs); }
   bool      _tap_wake = true;      // a touch wakes the dark screen (NVS)
   char      _pin_entry[9] = "";    // digits typed so far
   char      _pin_new[9] = "";      // setup: the first entry, waiting for its confirmation
-  uint8_t   _pin_fails = 0;
-  uint32_t  _pin_block_until = 0;  // millis() until which entry is refused after 5 misses
+  screenlock::Attempts _pin_tries;
 
   // Open conversation (SCR_THREAD)
   bool     _thread_is_channel = false;