feat(frontend): dashboard page with market / portfolio / alerts summary

[Shakalito]

Create /dashboard as the main authenticated home screen with high-level overview widgets.

AC:

• Market snapshot (basic prices or watchlist preview)

• Active alerts summary

• Quick navigation links (Portfolio, Alerts, Charts)

• Mobile-friendly stacked layout

• Proper loading and error handling

Deps

• feat(frontend): introduce public landing page and new routing structure #54

• feat(frontend): authenticated route guard with redirect-back #55