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.

docs/solo_features/settings_screen/settings_screen.md

@@ -98,7 +98,8 @@ The **repeater** mode and its flood filters live on their own screen — see **T
 
 | Setting  | Options    | Notes                                                                                              |
 | -------- | ---------- | -------------------------------------------------------------------------------------------------- |
-| Layout   | ABC / T9   | On-screen keyboard style. **ABC**: an a-b-c…z grid, one key per letter (the original layout). **T9**: phone-keypad multi-tap — each key is labelled with its **digit** and a letter group (e.g. `2abc`); repeated **Enter** presses cycle through the letters and then the digit itself. |
+| Layout   | ABC / T9   | On-screen keyboard style. **ABC**: an a-b-c…z grid, one key per letter (the original layout). **T9**: phone-keypad multi-tap — each key is labelled with its **digit** and a letter group (e.g. `2abc`); repeated **Enter** presses cycle through the letters and then the digit itself. Applies to whichever alphabet page is active (see Alphabet below), not just Latin. |
+| Alphabet | Latin / Cyrillic / Greek / Ext.Latin | Which extra (non-Latin) alphabet, if any, joins the keyboard's page cycle. **Latin** (default): only the Latin letters and Symbols pages, as before. Any other choice adds that alphabet's own page to the same **#@/abc** key's cycle (Latin → alphabet → Symbols → Latin) — no separate key to switch scripts. **Ext.Latin** is a curated set of Polish/Czech/Slovak/German/French/Spanish/Nordic diacritics (`ą ć ę ł ń ó ś ź ż`, `č š ž`, `ä ö ü ß`, `é è ê ë ñ`, etc.), not full Unicode coverage. **Greek** covers the 24-letter alphabet plus final sigma (`ς`) but not the tonos stress accents used in proper Modern Greek spelling. Typing in the chosen alphabet needs the **Lemon** font to actually display (Settings › Display › Font) — the keyboard forces it on temporarily while that page is open even if Font is set to Default, but received messages and everything else still follow your own Font choice. |
 
 Applies to every on-screen text field (messages, waypoint labels, room passwords, preset names). Earlier releases labelled the grid *QWERTY*; the layout has always been alphabetical, so it is now named **ABC**.