[6.x] Improve date UI bits

[jackmcdade]

[!NOTE] Polishes DatePicker/DateRangePicker UI (standardized trigger/clear buttons and input spacing) and adds a new text size token plus numeric alignment in the index field.

• UI Components
  • packages/ui/src/DatePicker/DatePicker.vue:
    • Standardize trigger and clear actions to Button (ghost/subtle) components; remove inline Icon usage.
    • Tweak container and input segment styling (alignment, spacing, colors); minor class cleanups.
  • packages/ui/src/DateRangePicker/DateRangePicker.vue:
    • Use Button for trigger and clear (subtle) actions; adjust trigger spacing.
    • Refine field layout: add items-center, text-md, tighter tracking, and reduce date segment padding (px-0.25).
• Styles
  • packages/ui/src/ui.css: Add --text-md size and --font-sans variables; minor theme additions.
• Control Panel
  • resources/js/components/fieldtypes/DateIndexFieldtype.vue: Add tabular-nums class for aligned numeric dates.

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[jasonvarga]

Ah sorry I merged #13088 not realizing this was open, and now there's a conflict.