Add single-day pages linked from timeline date headings

Each timeline date heading now links to /day/<YYYY-MM-DD>/, a page
holding just that day's photos with a back link to the timeline.
renderDayPages writes one per groupByDay run; the ISO label doubles
as the URL slug.

CLAUDE.md

@@ -48,6 +48,7 @@ Missing sidecar → filename as caption, today as date. Photos sort newest first
 - Two views of the same photos, both 3-column square grid → 2-column on mobile, both with infinite-scroll load-more. Page size is configurable via `-page-size` (default 60; NixOS `services.photogallery.pageSize`):
   - **Timeline is the site root** (`/`, `/page/2/`, …). Photos grouped by calendar day: one ISO date heading (`2026-06-14`) + count per day, then the square grid. `groupByDay` collapses the newest-first list into consecutive same-day runs; `chunkDayGroups` packs whole days into pages targeting `pageSize`, never splitting a day across a page boundary (so appended pages never duplicate a heading — the load-more JS relies on this).
   - **Grid** (`/grid/`, `/grid/page/2/`, …) is the flat square grid, linked from the profile header.
+  - Each timeline date heading links to a **single-day page** (`/day/<YYYY-MM-DD>/`) holding just that day's photos. `renderDayPages` writes one per `groupByDay` run; the ISO label doubles as the URL slug.
 - Single photo page with sidebar and newer/older navigation
 
 ### Photo page navigation
@@ -133,6 +134,7 @@ nix build          # succeeds
 | `assets.go` | `//go:embed` for templates and static |
 | `templates/timeline.html` | Site root: photos grouped by day, paginated |
 | `templates/grid.html` | Flat square grid at `/grid/`, paginated |
+| `templates/day.html` | Single-day page at `/day/<date>/` |
 | `templates/photo.html` | Single photo page, neighbour links and prefetch |
 | `static/photo-nav.js` | Arrow-key and swipe navigation |
 | `module.nix` | NixOS module (systemd service, tmpfiles, nginx vhost) |