Show placeholders in empty circuit lists

tests/circuit-grid-projection.test.ts

@@ -78,6 +78,41 @@ const sections: CircuitTreeSectionDto[] = [
 ];
 
 describe("circuit grid projection", () => {
+  it("keeps empty sections and their free placeholders when no filter is active", () => {
+    const emptySections: CircuitTreeSectionDto[] = [
+      {
+        id: "empty-lighting",
+        key: "lighting",
+        displayName: "Lighting",
+        prefix: "-1F",
+        sortOrder: 10,
+        circuits: [],
+      },
+      {
+        id: "empty-single-phase",
+        key: "single_phase",
+        displayName: "Single-phase circuits",
+        prefix: "-2F",
+        sortOrder: 20,
+        circuits: [],
+      },
+    ];
+
+    const projectedSections = filterAndSortCircuitSections(emptySections, {}, null);
+    const rows = buildVisibleGridRows(projectedSections);
+
+    assert.equal(projectedSections.length, 2);
+    assert.deepEqual(
+      rows.map((row) => row.rowKey),
+      [
+        "section:empty-lighting",
+        "placeholder:empty-lighting",
+        "section:empty-single-phase",
+        "placeholder:empty-single-phase",
+      ]
+    );
+  });
+
   it("keeps every device row in a circuit block when one device matches a filter", () => {
     const result = filterAndSortCircuitSections(sections, { roomNumberSnapshot: ["1.01"] }, null);