feat(reader): scroll 3/4 viewport on PgDn/PgUp

Adds PageDown/PageUp scrolling to the reader's scroll
container, sized at Math.round(clientHeight * 0.75) so the
user keeps ~25% overlap with the previously visible content
between presses (Vimium's d/u idiom).

- Reader.svelte: extends the existing $effect keydown
  listener; preventDefault + stopImmediatePropagation so the
  browser's default page-scroll doesn't double-fire.
- KeyboardHelp.svelte: documents PgDn/PgUp in the Reader
  section.

Author: antono

src/lib/components/KeyboardHelp.svelte

@@ -70,6 +70,7 @@
 			items: [
 				{ keys: [['j'], ['k']], desc: 'Next / previous message' },
 				{ keys: [['J'], ['K']], desc: 'Scroll down / up' },
+				{ keys: [['PgDn'], ['PgUp']], desc: 'Scroll 3/4 page down / up' },
 				{ keys: [['g', 'g']], desc: 'Scroll to top' },
 				{ keys: [['G']], desc: 'Scroll to bottom' },
 				{ keys: [['f']], desc: 'Follow link / attachment by hint' },

src/lib/components/Reader.svelte

@@ -70,6 +70,13 @@
 				e.preventDefault();
 				e.stopImmediatePropagation();
 				scrollEl.scrollBy({ top: (e.key === 'J' ? 1 : -1) * 20 * SCROLL_LINE_PX, behavior: 'smooth' });
+				return;
+			}
+			if (e.key === 'PageDown' || e.key === 'PageUp') {
+				e.preventDefault();
+				e.stopImmediatePropagation();
+				const step = Math.round(scrollEl.clientHeight * 0.75);
+				scrollEl.scrollBy({ top: e.key === 'PageDown' ? step : -step, behavior: 'smooth' });
 			}
 		}
 		window.addEventListener('keydown', onKeyDown);