feat: per-theme accent + Activity WhatsApp(en)/GitHub cards, softer icon borders

- theme: store accent separately for light/dark (theme-core/controls)
- en contacts: WhatsApp card instead of VK (wa.me link)
- Activity: add GitHub card (logo webp) in both locales
- mp-card__image border uses --line to match card (less contrast)

public/assets/shared/icons/whatsapp.svg

@@ -0,0 +1,4 @@
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