[6.x] Add option to show widget table headers

[jackmcdade]

Closes #14120

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Note

Low Risk
Primarily UI/styling changes plus a new optional widget config flag; low risk aside from potential minor visual regressions in widget tables.

Overview
Adds a new show_table_header widget config option for collection and form widgets, wiring it through PHP widget classes into the Vue components to optionally render visible table headers (instead of always sr-only).

Introduces shared .widget-table styling and updates widget tables/date cells to use consistent padding/alignment and numeric/date typography; Listing/TableHead.vue now exposes a data-column attribute on header cells to support column-specific header styling.

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