[6.x] Fix fields overflowing narrow groups

[eminos]

Fixes #15419.

A narrow Group can let its fields extend outside its assigned column, including when used inside a Bard set. For example, two full-width selects inside a 25% group beside a 75% textarea overflow instead of fitting the group.

The inner publish grid always defines 12 tracks, and its container queries resolve against a wider ancestor panel. The gaps alone can exceed the group's width. Once the group gets its own container, inherited --col-span values also need resetting so nested fields do not retain a parent's fractional span.

This change:

• Makes the Group's inner wrapper a named panel container, so its fields respond to the group's available width.
• Uses one grid track below the existing @lg/panel breakpoint and 12 tracks above it.
• Resets --col-span at each publish/field grid so nested grids calculate their own spans.

Narrow groups stack their fields; wider groups retain configured column widths. This changes layout only, with no blueprint or stored-value migration.

Tests

Adds a Vitest browser project using the existing Playwright dependency. These tests mount the core publish and fieldtype components with the actual Control Panel CSS so assertions measure browser layout, which jsdom cannot provide. The existing CI command runs the new project alongside unit and Storybook tests.

The 20 cases cover the textarea/select reproduction, several group and panel widths, the 511/512px breakpoint, nested groups, bordered and borderless groups, fullscreen transitions, collapse/expand, spacers, and nested form-value updates.

• Unpatched current 6.x: 11 layout tests fail, 9 pass.
• Patched branch: npm run test passes all 894 tests across 117 files.
• npm run build passes.
• Manually verified nested Bard/group layouts and fullscreen behavior in the Control Panel.

Prepared with AI assistance; the patch was inspected and tested locally before submission.

[jasonvarga]

Looks ok to me. @jaygeorge can you give the css parts of this a once-over?

[jaygeorge]

The CSS part looks good to me.