feat(ui/clock): stacked big-digit clock on portrait e-ink

On a tall portrait e-ink panel the clock and lock screens wasted most of
the vertical space on a small inline "HH:MM". Render HH and MM on two
lines in a new size-4 font (built-in GFX scaled 7×, ~42×56 px) so the
digits roughly double in height and fill the narrow width. Wide panels
(OLED, landscape e-ink) keep the classic single-line size-2 layout.

- New shared drawClockTime() helper used by both the Clock home page and
  the lock screen; returns the y below the time block so the date and
  dashboard rows flow beneath it.
- GxEPDDisplay: size 4 = built-in font × BIG_TEXT_SCALE (7); getCharWidth /
  getLineHeight / setTextSize handle it; fontAscender stays 0 (built-in is
  top-left origin).
- Centre the big digits on their visible glyph width — the built-in font
  advances 6 px per char but the glyph is 5 px wide, so getTextWidth over-
  reports by one trailing column and the digits would sit ~half a column
  left of centre. AM/PM rendered one size larger (size 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/helpers/ui/GxEPDDisplay.cpp

@@ -141,6 +141,12 @@ void GxEPDDisplay::setTextSize(int sz) {
   // Size 3 always uses FreeSans18pt for large headings.
   int sc = scale();
   switch (sz) {
+    case 4:
+      // Huge clock digits: built-in font scaled up. Cursor stays top-left
+      // (fontAscender returns 0 for the built-in font), so layout maths is plain.
+      display.setFont(NULL);
+      display.setTextSize(BIG_TEXT_SCALE);
+      break;
     case 3:
       display.setFont(&FreeSans18pt7b);
       display.setTextSize(1);