docs nav: correct casing of 'Searching and Filtering' sidebar label

docTitleOverrides now pins the label with lowercase 'and' (the naive
titleCase fallback capitalized every word); only the sidebar label was
affected — the page h1 comes from the markdown heading and was already
correct. Test pin added to TestDocTitle.

internal/docs/navigation.go

@@ -47,6 +47,7 @@ var docTitleOverrides = map[string]string{
 	"FRONTEND_INTEGRATION.md":                   "Frontend Integration",
 	"user/user-guide.md":                        "User Guide",
 	"user/sync-guide.md":                        "Sync Guide",
+	"user/searching-and-filtering.md":           "Searching and Filtering",
 	"user/devices/kobo-setup.md":                "Kobo Setup",
 	"user/devices/koreader-setup.md":            "KOReader Setup",
 	"user/devices/android-app.md":               "Android App",

internal/docs/navigation_test.go

@@ -50,6 +50,7 @@ func TestDocTitle(t *testing.T) {
 	tests := map[string]string{
 		"index.md":                                 "Overview",
 		"user/user-guide.md":                       "User Guide",
+		"user/searching-and-filtering.md":          "Searching and Filtering",
 		"user/devices/koreader-setup.md":           "KOReader Setup",
 		"developer/development.md":                 "Developer Portal",
 		"developer/api/api-reference.md":           "Overview",