Field-group horizontal-tabs wrapper has no card chrome on entity view displays

Problem/Motivation

On entity edit forms, Gin styles Field Group's horizontal-tabs widget so the subtabs
sit inside a contained card, visually separated from the page's primary task tabs above. On entity
view displays the same Field Group tabs render flat, without that separation.

This is confusing when both task tabs and field-group subtabs are visible on the same page (e.g. the
user view page with field groups configured on the view display) — the field-group tab strip blends into
the task tabs above it, and it's not obvious which set of tabs you're looking at.

Steps to reproduce

1. Install Gin and Field Group.
2. On an entity type (e.g. User), configure the default view display to wrap fields in a
   Horizontal Tabs field group.
3. View an entity of that type as a user who also sees task tabs (View / Edit / etc.).
4. Compare with the equivalent edit form, where the field-group tabs sit inside a styled card.

Proposed resolution

Apply the same card chrome to .field-group-tabs-wrapper unconditionally, so the visual
treatment of view and edit matches across all entity types. In dark mode, use --gin-bg-layer3
rather than --gin-bg-layer2, because bg-layer2 shares its value with
--gin-bg-input and would make unchecked toggle switches (and other inputs) invisible against
the card.

Working CSS from a Gin sub-theme:

.field-group-tabs-wrapper {
    background-color: var(--gin-bg-layer);
    border: 1px solid var(--gin-border-color);
    border-radius: var(--gin-border-l);
    padding: var(--gin-spacing-m) var(--gin-spacing-l);
    margin-block-start: var(--gin-spacing-m);
  }

  /* Pull horizontal-tabs back inside the card padding so its tab strip aligns
     with the card edge rather than indenting from it. */
  .field-group-tabs-wrapper .horizontal-tabs {
    margin-block-start: 0;
  }

  /* Dark mode: bg-layer is too close to the page background, but bg-layer2 is
     the same value as --gin-bg-input — using it would make unchecked toggle
     switches (and other inputs) invisible against the card. bg-layer3 sits
     one shade above the input background. */
  .gin--dark-mode .field-group-tabs-wrapper {
    background-color: var(--gin-bg-layer3);
  }

Remaining tasks

• Confirm the approach (unconditional styling vs. scoping to non-edit contexts).
• Roll into Gin's existing field-group / horizontal-tabs CSS.
• Open an MR against 5.x.
• Figure out how we handle this in 6.x/core.

User interface changes

Field-group horizontal-tabs on entity view displays gain the same card chrome already used on entity
edit forms. No change to edit-form rendering.