Add -default-view to choose timeline or grid as the site root

The view named by -default-view (timeline, the default, or grid) is published
at the site root; the other is served under /timeline/ or /grid/. viewBase and
pageURL derive every page path, pager link and the Timeline<->Grid cross-link
from the single setting, and Build removes both timeline/ and grid/ on each run
so flipping the default never leaves a stale root behind.

Reimplemented on trunk (which has albums and the /manage feature) rather than
cherry-picked, since the original was written against an earlier base. Credit
for the edit/delete and default-view ideas goes to Santhosh Raju, now noted in
a README Contributors section.

- main.go: -default-view flag, validated (fatal on anything but timeline/grid)
- module.nix: services.photogallery.defaultView enum option
- templates: timeline/grid/day/album/albums cross-links use TimelineURL/GridURL
- tests: timeline-root, grid-root, and switch-cleanup

CLAUDE.md

@@ -113,9 +113,9 @@ contain spaces (`"berlin trip"`), and its URL slug is derived with `sanitiseSlug
 - 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`
-- 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.
+- 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`). Which view sits at the site root is chosen by `-default-view` (`timeline` or `grid`, default `timeline`; NixOS `services.photogallery.defaultView`): the chosen one owns `/` (`/`, `/page/2/`, …) and the other is published under `/<name>/`. `viewBase`/`pageURL` (`generator.go`) derive every page path, pager link and the Timeline↔Grid cross-links (`TimelineURL`/`GridURL` in the template data) from that one setting; `Build` removes both `timeline/` and `grid/` each run so a flipped default leaves no stale root behind.
+  - **Timeline** (the default root): 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** is the flat square grid. At `/grid/` by default, or at the root when `-default-view grid`.
   - 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