Reorganize source-strategy settings in the responsive video style form

Problem/Motivation

When configuring a responsive video style that uses Multiple sources in one video, the form currently mixes shared video settings and responsive source mappings in a way that is less clear than the corresponding video_style UI.

The current form does not make poster-selection behavior very discoverable, and the order of the source-strategy-specific settings does not align well with the existing single video style forms. This makes the form feel heavier and less predictable for site builders.

The source-strategy form should present its shared settings more clearly and use a section structure that is easier to scan and more consistent with the rest of the Video style UI.

Steps to reproduce

1. Install responsive_video_style.
2. Create or edit a responsive video style.
3. Select Multiple sources in one video as the responsive strategy.
4. Review the order and grouping of the source-strategy-specific sections.
5. Observe that dimensions, poster behavior, and video tag settings are not presented as clearly or as consistently as in the single video style form.

Proposed resolution

Reorganize the source-strategy-specific sections of the responsive video style form so the UI is clearer and more consistent with video_style.

For Multiple sources in one video, the form should use this order:

1. Dimensions
2. Breakpoint mappings
3. Poster image settings
4. <video> tag settings

The form should also:

• show Dimensions as expanded by default,
• move poster-selection guidance into its own Poster image settings details section, and
• avoid hiding that behavior inside the fallback-source description.

The poster settings section does not need editable fields yet. It should explain the current behavior:

• the rendered video uses one shared poster image,
• configured fallback video styles are checked first, and
• if none provides a poster, mapped video styles are checked in order.

Remaining tasks

• Reorder the source-strategy-specific sections in the responsive video style form.
• Add a dedicated Poster image settings details section for source-strategy forms.
• Move the poster-selection explanation into that section.
• Verify that the resulting form still rebuilds correctly for responsive strategy changes and breakpoint mapping interactions.

User interface changes

Yes.

The Multiple sources in one video form layout changes. It gets a dedicated Poster image settings section, and the source-strategy-specific sections are reordered for better consistency and readability.

API changes

No.

This issue changes admin form structure and help text only.

Data model changes

No.

This issue does not change stored configuration or schema.