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

albums.go

@@ -108,6 +108,8 @@ func (g *Generator) baseData() map[string]any {
 		"FontsURL":    g.fontsURL,
 		"SerifFamily": g.serifFamily,
 		"MlFamily":    g.mlFamily,
+		"TimelineURL": g.timelineURL(),
+		"GridURL":     g.gridURL(),
 	}
 }