Make the timeline the default view, paginated and configurable

The day-grouped timeline is now the site root (/, /page/N/); the flat
square grid moves to /grid/ (/grid/page/N/). Both are linked from the
profile header, separated by a divider.

Timeline pages pack whole day-groups until they reach the page size,
never splitting a day across a page boundary, so appended pages never
repeat a date heading — the infinite-scroll handler relies on this.

Page size is configurable via -page-size (default 60), threaded through
the Generator and exposed as services.photogallery.pageSize in the
NixOS module.

static/style.css

@@ -130,6 +130,16 @@ body {
   color: var(--fg2);
 }
 
+/* Divider between adjacent header links, e.g. GRID | RSS FEED. The pipe is a
+   flex child of the second link, so its right margin adds to that link's
+   gap:5px — 9px + 5px matches the 14px left margin for even spacing. */
+.rss-a + .rss-a::before {
+  content: "|";
+  margin-left: 14px;
+  margin-right: 9px;
+  color: var(--fg4);
+}
+
 /* ── Photo grid ────────────────────────────────────────────── */
 
 .grid-wrap {