Does not inform inner stack about a Panel mode

[ildar170975]

In Panel mode, stock cards (vertical-stack, horizontal-stack, grid) use a "ispanel" property to style themself properly.
For instance, this defines processing of borders & box-shadows:
https://github.com/home-assistant/frontend/blob/79ac2a72fa39636e628b10b7bd04dad7a4b59a15/src/panels/lovelace/cards/hui-stack-card.ts#L109

      :host([ispanel]) #root {
        --ha-card-border-radius: var(--restore-card-border-radius);
        --ha-card-border-width: var(--restore-card-border-width);
        --ha-card-box-shadow: var(--restore-card-border-shadow);
      }

When auto-entities is used with stacks in a Panel mode - it causes absence of borders.
Consider these 2 cards:
-- grid with auto-entities
-- grid

type: custom:auto-entities
card:
  type: grid
  square: false
  columns: 2
card_param: cards
filter:
  include:
    - &ref_card_1
      entity_id: sun.sun
      options:
        type: entity
        entity: this.entity_id
    - *ref_card_1
    - *ref_card_1
    - *ref_card_1

type: grid
square: false
columns: 2
cards:
  - &ref_card_2
    type: entity
    entity: sun.sun
  - *ref_card_2
  - *ref_card_2
  - *ref_card_2

Both cards properly shown in a masonry layout:

The 1s card (with auto-entities) in Panel mode - no borders are shown:

The 2nd card in Panel mode - borders are shown: