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

default_view_test.go

@@ -0,0 +1,108 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// buildWithView builds a two-photo site with the given default view and returns
+// the output dir.
+func buildWithView(t *testing.T, view string) string {
+	t.Helper()
+	content := t.TempDir()
+	output := t.TempDir()
+	writePNG(t, filepath.Join(content, "a.png"))
+	writePNG(t, filepath.Join(content, "b.png"))
+
+	g := NewGenerator(content, output, "http://x", "A", "@a", "bio", "", "f", "s", "m", 60, view)
+	if err := g.Build(); err != nil {
+		t.Fatalf("build (%s): %v", view, err)
+	}
+	return output
+}
+
+func exists(p string) bool { _, err := os.Stat(p); return err == nil }
+
+func TestDefaultViewTimeline(t *testing.T) {
+	out := buildWithView(t, viewTimeline)
+
+	// Timeline owns the root; grid is the secondary under /grid/.
+	if !exists(filepath.Join(out, "index.html")) {
+		t.Error("root index.html missing")
+	}
+	if !exists(filepath.Join(out, "grid", "index.html")) {
+		t.Error("/grid/ should exist as the secondary view")
+	}
+	// No /timeline/ dir — timeline is at the root, not under its own name.
+	if exists(filepath.Join(out, "timeline")) {
+		t.Error("/timeline/ should not exist when timeline is the default")
+	}
+
+	// The root (timeline) links to grid at /grid/.
+	root := readFile(t, filepath.Join(out, "index.html"))
+	if !strings.Contains(root, `href="/grid/"`) {
+		t.Error("timeline root should link to /grid/")
+	}
+}
+
+func TestDefaultViewGrid(t *testing.T) {
+	out := buildWithView(t, viewGrid)
+
+	// Grid owns the root; timeline is the secondary under /timeline/.
+	if !exists(filepath.Join(out, "index.html")) {
+		t.Error("root index.html missing")
+	}
+	if !exists(filepath.Join(out, "timeline", "index.html")) {
+		t.Error("/timeline/ should exist as the secondary view")
+	}
+	// No /grid/ dir — grid is at the root now.
+	if exists(filepath.Join(out, "grid")) {
+		t.Error("/grid/ should not exist when grid is the default")
+	}
+
+	// The root (grid) links back to the timeline at /timeline/.
+	root := readFile(t, filepath.Join(out, "index.html"))
+	if !strings.Contains(root, `href="/timeline/"`) {
+		t.Error("grid root should link to /timeline/")
+	}
+}
+
+// TestDefaultViewSwitchCleansUp confirms that flipping the default view does not
+// leave the previous root's view directory behind.
+func TestDefaultViewSwitchCleansUp(t *testing.T) {
+	content := t.TempDir()
+	output := t.TempDir()
+	writePNG(t, filepath.Join(content, "a.png"))
+
+	// First build with grid as root → creates /timeline/.
+	g1 := NewGenerator(content, output, "http://x", "A", "@a", "b", "", "f", "s", "m", 60, viewGrid)
+	if err := g1.Build(); err != nil {
+		t.Fatal(err)
+	}
+	if !exists(filepath.Join(output, "timeline")) {
+		t.Fatal("expected /timeline/ after grid-default build")
+	}
+
+	// Rebuild with timeline as root → /timeline/ must be gone, /grid/ present.
+	g2 := NewGenerator(content, output, "http://x", "A", "@a", "b", "", "f", "s", "m", 60, viewTimeline)
+	if err := g2.Build(); err != nil {
+		t.Fatal(err)
+	}
+	if exists(filepath.Join(output, "timeline")) {
+		t.Error("/timeline/ left over after switching default to timeline")
+	}
+	if !exists(filepath.Join(output, "grid", "index.html")) {
+		t.Error("/grid/ should exist after switching default to timeline")
+	}
+}
+
+func readFile(t *testing.T, p string) string {
+	t.Helper()
+	b, err := os.ReadFile(p)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return string(b)
+}