added scrollspy for TOC highlighting, theme toggle support, and dark/light mode handling

site/src/jsTest/kotlin/ScrollSpyTest.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2025 Sergey S. Chernov real.sergeych@gmail.com
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class ScrollSpyTest {
+    @Test
+    fun emptyListReturnsZero() {
+        assertEquals(0, activeIndexForTops(emptyList(), 80.0))
+    }
+
+    @Test
+    fun selectsFirstWhenOnlyFirstIsAboveOffset() {
+        val tops = listOf(20.0, 200.0, 800.0) // px from viewport top
+        val idx = activeIndexForTops(tops, offsetPx = 80.0)
+        assertEquals(1, idx) // 20 <= 80, 200 > 80 -> index 1 (second heading is first below offset)
+    }
+
+    @Test
+    fun selectsLastHeadingAboveOffset() {
+        val tops = listOf(-100.0, 50.0, 70.0)
+        val idx = activeIndexForTops(tops, offsetPx = 80.0)
+        assertEquals(2, idx) // all three are <= 80 -> last index
+    }
+
+    @Test
+    fun stopsBeforeFirstBelowOffset() {
+        val tops = listOf(-200.0, -50.0, 30.0, 150.0, 400.0)
+        val idx = activeIndexForTops(tops, offsetPx = 80.0)
+        assertEquals(3, idx) // 30 <= 80 qualifies; 150 > 80 stops, so index 3rd (0-based -> 3?)
+    }
+}