feat(web): support list editing for channel array fields (#2595)

Add reusable channel array list controls and parsing utilities for channel forms.
Normalize channel string-array payloads in the backend, including pasted values,
numeric IDs, hidden characters, duplicates, and empty clears.
Also allow FlexibleStringSlice to unmarshal null values and cover the new behavior
with backend and config tests.

pkg/config/config_struct.go

@@ -22,6 +22,11 @@ import (
 type FlexibleStringSlice []string
 
 func (f *FlexibleStringSlice) UnmarshalJSON(data []byte) error {
+	if string(data) == "null" {
+		*f = nil
+		return nil
+	}
+
 	// Accept a single JSON string for convenience, e.g.:
 	// "text": "Thinking..."
 	var singleString string

pkg/config/config_test.go

@@ -1258,6 +1258,11 @@ func TestFlexibleStringSlice_UnmarshalJSON(t *testing.T) {
 		input    string
 		expected []string
 	}{
+		{
+			name:     "null",
+			input:    `null`,
+			expected: nil,
+		},
 		{
 			name:     "single string",
 			input:    `"Thinking..."`,
@@ -1286,6 +1291,12 @@ func TestFlexibleStringSlice_UnmarshalJSON(t *testing.T) {
 			if err := json.Unmarshal([]byte(tt.input), &f); err != nil {
 				t.Fatalf("json.Unmarshal(%s) error = %v", tt.input, err)
 			}
+			if tt.expected == nil {
+				if f != nil {
+					t.Fatalf("json.Unmarshal(%s) = %#v, want nil slice", tt.input, f)
+				}
+				return
+			}
 			if len(f) != len(tt.expected) {
 				t.Fatalf("json.Unmarshal(%s) len = %d, want %d", tt.input, len(f), len(tt.expected))
 			}