SubmissionDetails view should honor row/column/cell layout

[harshtandiya]

Context

Forms now support a 3-axis layout (row_index, column_index, cell_index) on FormField. FormRenderer.vue honors this for new submissions via the public form. SubmissionDetails.vue (the read-only detail view of an existing submission) still renders fields as a flat list.

Expected

SubmissionDetails.vue should match FormRenderer.vue's 3D structure:

• Outer <template v-for> over rows
• Inner over columns (flex-col)
• Innermost over cells (already vertical)
• flex-col md:flex-row gap-4 at row level for mobile stack
• flex-1 min-w-0 on field wrapper

Reference

• frontend/src/components/submission/FormRenderer.vue for the 3D pattern
• frontend/src/composables/useGroupedRows.ts returns FormField[][][]

Out of scope

• Editing layout from the details view (it's read-only).