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);

src/helpers/ui/GxEPDDisplay.h

@@ -93,16 +93,21 @@ public:
   //   1 = FreeSans9pt      (lineH=16, charW≈9)
   //   2 = FreeSansBold12pt (lineH=20, charW≈12)
   //   3 = FreeSans18pt     (lineH=28, charW≈17)
+  // Built-in font scale used for size 4 (huge clock digits): 6×8 cell × 7 = 42×56.
+  static const int BIG_TEXT_SCALE = 7;
   // Size 1 scales with orientation (portrait 1×, landscape 2×); size 2 is always 12×16;
-  // size 3 is FreeSans18pt (~17×28). Landscape = width >= height.
+  // size 3 is FreeSans18pt (~17×28); size 4 is the built-in font at 7× (~42×56),
+  // used only for the stacked HH/MM clock on portrait e-ink. Landscape = width >= height.
   int getCharWidth() const override {
     int sc = scale();
+    if (_text_sz == 4) return 6 * BIG_TEXT_SCALE;
     if (_text_sz == 3) return 17;
     if (_text_sz == 2) return 12 * sc;
     return (_use_lemon ? 5 : 6) * sc;
   }
   int getLineHeight() const override {
     int sc = scale();
+    if (_text_sz == 4) return 8 * BIG_TEXT_SCALE;
     if (_text_sz == 3) return 28;
     if (_text_sz == 2) return 16 * sc;
     return (_use_lemon ? 10 : 8) * sc;