Add selected state and hover background to Card

- Add `selected` prop: brand border (warm gold) + warm background tint (brand.50)
- Add hover background fill (neutral.50) for interactive cards
- Add 3 new card tokens: border.selected, background.hover, background.selected
- Add stories: Selected, OptionSelect (single-select), MultiSelect (toggle),
  OnDifferentBackgrounds (white vs grey surface comparison)
- Informed by FA 1.0 Figma ListItemPurchaseOption pattern (node 2349:39505)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tokens/component/card.json

@@ -19,13 +19,16 @@
     },
     "border": {
       "$type": "color",
-      "$description": "Border colours for the outlined card variant.",
-      "default": { "$value": "{color.border.default}", "$description": "Neutral border for outlined cards" }
+      "$description": "Border colours for card variants and states.",
+      "default": { "$value": "{color.border.default}", "$description": "Neutral border for outlined cards" },
+      "selected": { "$value": "{color.border.brand}", "$description": "Brand border for selected/active cards — warm gold accent" }
     },
     "background": {
       "$type": "color",
       "$description": "Card background colours.",
-      "default": { "$value": "{color.surface.raised}", "$description": "White — standard card background (raised surface)" }
+      "default": { "$value": "{color.surface.raised}", "$description": "White — standard card background (raised surface)" },
+      "hover": { "$value": "{color.surface.subtle}", "$description": "Subtle grey fill on hover — neutral.50 for soft interactive feedback" },
+      "selected": { "$value": "{color.surface.warm}", "$description": "Warm tint for selected cards — brand.50 reinforces active state" }
     }
   }
 }