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.

CLAUDE.md

@@ -45,9 +45,10 @@ Missing sidecar → filename as caption, today as date. Photos sort newest first
 - Gruvbox dark hard colour scheme
 - Crimson Pro (serif) for headings and captions — configurable via `-serif-family` / `-fonts-url`
 - Manjari for Malayalam text (bio line), via `-ml-family`
-- 3-column square grid → 2-column on mobile, 60 photos per page (`/`, `/page/2/`, …)
+- 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.
 - Single photo page with sidebar and newer/older navigation
-- Timeline page (`/timeline/`) groups photos by calendar day: one ISO date heading (`2026-06-14`) + count per day, then the same square grid. Linked from the profile header; `groupByDay` in `generator.go` collapses the newest-first photo list into consecutive same-day runs, so it is not paginated.
 
 ### Photo page navigation
 
@@ -130,8 +131,9 @@ nix build          # succeeds
 | `server.go` | Upload API + optional static file serving |
 | `watcher.go` | fsnotify watcher with 500ms debounce |
 | `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/photo.html` | Single photo page, neighbour links and prefetch |
-| `templates/timeline.html` | Timeline page, photos grouped by day |
 | `static/photo-nav.js` | Arrow-key and swipe navigation |
 | `module.nix` | NixOS module (systemd service, tmpfiles, nginx vhost) |
 | `flake.nix` | `buildGoModule` package + devShell + nixosModules |