feat(clock,keyboard): alarm repeat + non-Latin keyboard alphabets

Alarm repeat (Clock Tools):
- NodePrefs::alarm_repeat_mask (weekday bitmask, struct tm::tm_wday
  convention) — new Repeat row cycles Off/Daily/Weekdays/Weekends.
  computeAlarmNextFire() scans the next 7 days for a matching weekday when
  set; evaluateAlarm() only clears alarm_on (one-shot) when the mask is
  empty, otherwise re-arms. mask==0 is byte-for-byte the original one-shot
  behaviour, so existing users see no change.

On-screen keyboard alphabets (Settings > Keyboard > Alphabet):
- KeyboardWidget reworked from single-byte ASCII cells to UTF-8 codepoints
  (insertion, backspace and T9 in-place cycling all codepoint-aware now, so a
  multi-byte character is never split) — see kbApplyCapsUtf8/kbUtf8Len/
  kbUtf8CharAt/kbUtf8LastCharBytes.
- Cyrillic, Greek and an Extended Latin set (Polish/Czech/Slovak/German/
  French/Spanish/Nordic diacritics) join the keyboard's existing #@/abc page
  cycle (Latin -> alt alphabet -> Symbols -> Latin) — no new key needed.
  NodePrefs::keyboard_alt_alphabet picks which one, if any, is active.
- Lemon font is forced on transiently inside KeyboardWidget::render() (saved
  and restored every call) whenever the alt-alphabet page is showing or
  already-typed text has non-ASCII bytes, so composing is visible even if
  Font is set to Default.
- Each script's caps-shift rule is distinct and documented in
  kbApplyCapsUtf8: flat -0x20 for ASCII/Cyrillic/Greek (with the ё/ς
  exceptions), flat -0x20 for Latin-1 (à-þ), and an adjacent-pair -1 for
  Latin Extended-A (verified against every character actually used, not a
  blanket rule for that whole Unicode block).

NodePrefs schema: two tail-appended fields this session (alarm_repeat_mask,
keyboard_alt_alphabet), SCHEMA_SENTINEL 0xC0DE001B -> 0xC0DE001D,
sizeof(NodePrefs) 2496 -> 2504 (verified via a standalone host compile).

Docs: Clock Tools' Repeat row, Settings > Keyboard > Alphabet, and the
Rooms keyboard note (no longer ASCII-only once an alphabet is enabled).

Not build-verified — no PlatformIO toolchain available this session. Caps
mappings cross-checked against Python's Unicode case tables; UTF-8 literal
bytes verified at the byte level.

examples/companion_radio/DataStore.cpp

@@ -482,6 +482,19 @@ void DataStore::loadPrefsInt(const char *filename, NodePrefs& _prefs, double& no
   rd(&_prefs.trail_autosave_lowbatt, sizeof(_prefs.trail_autosave_lowbatt));
   if (_prefs.trail_autosave_lowbatt > 1) _prefs.trail_autosave_lowbatt = 0;
 
+  // → 0xC0DE001C: append alarm_repeat_mask at the tail. A pre-0x1C file has the
+  // old sentinel bytes / EOF here; any value that isn't one of the four presets
+  // clamps to 0 (no repeat / one-shot), matching the original alarm behaviour
+  // upgraders already had.
+  rd(&_prefs.alarm_repeat_mask, sizeof(_prefs.alarm_repeat_mask));
+  if (NodePrefs::alarmRepeatIdxForMask(_prefs.alarm_repeat_mask) == 0) _prefs.alarm_repeat_mask = 0;
+
+  // → 0xC0DE001D: append keyboard_alt_alphabet at the tail. A pre-0x1D file has
+  // the old sentinel bytes / EOF here; clamp anything out of range to 0 (Latin
+  // only), matching the keyboard's original (Latin-only) behaviour.
+  rd(&_prefs.keyboard_alt_alphabet, sizeof(_prefs.keyboard_alt_alphabet));
+  if (_prefs.keyboard_alt_alphabet >= NodePrefs::KB_ALPHABET_COUNT) _prefs.keyboard_alt_alphabet = 0;
+
   // Schema sentinel: bumped on layout changes. Mismatch means an older file
   // (or a different schema); rd() already zero-inits any fields not present,
   // so we just log it — next savePrefs writes the current sentinel.
@@ -668,6 +681,8 @@ void DataStore::savePrefs(const NodePrefs& _prefs, double node_lat, double node_
     file.write((uint8_t *)&_prefs.page_order[NodePrefs::PAGE_ORDER_LEN_V1],
                NodePrefs::PAGE_ORDER_LEN - NodePrefs::PAGE_ORDER_LEN_V1);
     file.write((uint8_t *)&_prefs.trail_autosave_lowbatt, sizeof(_prefs.trail_autosave_lowbatt));
+    file.write((uint8_t *)&_prefs.alarm_repeat_mask,      sizeof(_prefs.alarm_repeat_mask));
+    file.write((uint8_t *)&_prefs.keyboard_alt_alphabet,  sizeof(_prefs.keyboard_alt_alphabet));
 
     // Tail sentinel — must be last. See NodePrefs::SCHEMA_SENTINEL. Its write is
     // the one we check: once the flash fills, writes return 0, so a good