fix(api): return empty arrays not null for empty lists (frontend null.length crash)

Root cause: store List* methods used 'var out []T' which stays nil on empty
result sets and serializes to JSON null; the SPA then crashed on .length/.map
(e.g. endpoints.length on the Tasks page right after deploy). Return []T{}
at the source; coerce null->[] on the frontend load sites as defense-in-depth.
Regression test asserts List* are non-nil when empty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

internal/store/accounts.go

@@ -38,7 +38,7 @@ func (s *Store) ListAccountsByTask(ctx context.Context, taskID int64) ([]Account
 		return nil, err
 	}
 	defer rows.Close()
-	var out []Account
+	out := []Account{}
 	for rows.Next() {
 		var a Account
 		if err := rows.Scan(&a.ID, &a.TaskID, &a.SrcLogin, &a.SrcPassEnc, &a.DstLogin, &a.DstPassEnc,

internal/store/emptylist_test.go

@@ -0,0 +1,38 @@
+package store
+
+import (
+	"context"
+	"testing"
+)
+
+// Empty lists must serialize as JSON [] (non-nil slice), not null,
+// otherwise the frontend crashes on `.length`/`.map`. Regression for
+// "Uncaught TypeError: can't access property length, n is null".
+func TestListsNeverNilWhenEmpty(t *testing.T) {
+	s := testStore(t)
+	ctx := context.Background()
+
+	eps, err := s.ListEndpoints(ctx)
+	if err != nil {
+		t.Fatalf("ListEndpoints: %v", err)
+	}
+	if eps == nil {
+		t.Fatal("ListEndpoints returned nil slice on empty DB (would serialize as null)")
+	}
+
+	tasks, err := s.ListTasks(ctx)
+	if err != nil {
+		t.Fatalf("ListTasks: %v", err)
+	}
+	if tasks == nil {
+		t.Fatal("ListTasks returned nil slice on empty DB (would serialize as null)")
+	}
+
+	accs, err := s.ListAccountsByTask(ctx, 999999)
+	if err != nil {
+		t.Fatalf("ListAccountsByTask: %v", err)
+	}
+	if accs == nil {
+		t.Fatal("ListAccountsByTask returned nil slice for no rows (would serialize as null)")
+	}
+}

internal/store/endpoints.go

@@ -34,7 +34,7 @@ func (s *Store) ListEndpoints(ctx context.Context) ([]Endpoint, error) {
 		return nil, err
 	}
 	defer rows.Close()
-	var out []Endpoint
+	out := []Endpoint{}
 	for rows.Next() {
 		var e Endpoint
 		if err := rows.Scan(&e.ID, &e.RoleLabel, &e.Host, &e.Port, &e.TLSMode); err != nil {

internal/store/tasks.go

@@ -40,7 +40,7 @@ func (s *Store) ListTasks(ctx context.Context) ([]Task, error) {
 		return nil, err
 	}
 	defer rows.Close()
-	var out []Task
+	out := []Task{}
 	for rows.Next() {
 		var t Task
 		if err := rows.Scan(&t.ID, &t.Name, &t.SrcEndpointID, &t.DstEndpointID, &t.Status, &t.FolderMapping); err != nil {

web/src/pages/Endpoints.tsx

@@ -11,7 +11,7 @@ export function Endpoints() {
 
   function reload() {
     listEndpoints()
-      .then(setEndpoints)
+      .then((e) => setEndpoints(e ?? []))
       .catch((e) => setError(String(e.message || e)))
   }
 

web/src/pages/Tasks.tsx

@@ -13,13 +13,13 @@ export function Tasks() {
 
   function reload() {
     listTasks()
-      .then(setTasks)
+      .then((t) => setTasks(t ?? []))
       .catch((e: unknown) => setError(e instanceof Error ? e.message : 'Failed to load tasks'))
   }
 
   useEffect(() => {
     reload()
-    listEndpoints().then(setEndpoints).catch(() => {})
+    listEndpoints().then((e) => setEndpoints(e ?? [])).catch(() => {})
   }, [])
 
   async function submit(e: FormEvent) {