fix(docs): folder-index pages expand the sidebar and highlight — same rule for every page

The filter manual is the docs tree's first FOLDER-index page: it serves
at /docs/user/filters while its nav entry carries /index
(/docs/user/filters/index). The sidebar's two checks — group expansion
(containsURL) and active-link styling (docsNavClass) — compared URLs
EXACTLY, so the folder URL matched neither: no submenu expansion, no
highlight (2026-11-01 user report: "the left sidebar should open to
the submenu and highlight the page").

Fix is structural, not per-page: both checks normalize folder-index
URLs (a trailing /index is the same page as its directory), so EVERY
folder-index page — current and future — expands and highlights like
a file page. No per-page registration exists anywhere in the docs
system to redo: the sidebar derives from the docs tree, and this was
the one URL shape its equality rule missed.

Pins: navigation (section + group expand for the folder URL, group
present) and docsNavClass (folder URL highlights the Overview item,
normalization symmetric, file pages keep their highlight, no
cross-highlighting).

internal/docs/navigation_test.go

@@ -4,6 +4,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"bookhoard/templates"
 )
 
 func TestPlaceDoc(t *testing.T) {
@@ -205,3 +207,41 @@ func TestBuildNavigationNothingCurrent(t *testing.T) {
 		t.Errorf("expected zero sections open when nothing matches (docs home / 404 renders fully closed), got %d", open)
 	}
 }
+
+// TestBuildNavigationFolderIndex (2026-11-01): folder-index pages serve
+// at their directory URL (/docs/user/filters) while their nav entries
+// carry /index (/docs/user/filters/index) — the sidebar must EXPAND the
+// containing section/group and HIGHLIGHT the index item for the folder
+// URL, exactly like a file page. The filter manual's folder is the
+// first page of this shape.
+func TestBuildNavigationFolderIndex(t *testing.T) {
+	// The REAL docs tree: the Filters group only exists because the
+	// filter manual (guide/cookbook/fields) lives in user/filters/.
+	handler := NewDocsHandler("../../docs")
+	nav := handler.BuildNavigation("/docs/user/filters")
+
+	var userGuide *templates.NavSection
+	for i := range nav.Sections {
+		if nav.Sections[i].Title == "User Guide" {
+			userGuide = &nav.Sections[i]
+		}
+	}
+	if userGuide == nil {
+		t.Fatal("User Guide section missing")
+	}
+	if userGuide.Collapsed {
+		t.Error("User Guide section must expand for /docs/user/filters")
+	}
+	var filters *templates.NavGroup
+	for i := range userGuide.Groups {
+		if userGuide.Groups[i].Title == "Filters" {
+			filters = &userGuide.Groups[i]
+		}
+	}
+	if filters == nil {
+		t.Fatal("Filters group missing from User Guide")
+	}
+	if filters.Collapsed {
+		t.Error("Filters group must expand for its folder URL")
+	}
+}