fix: stabilize dashboard authentication and standardize api responses

[T9ner]

Resolves issues found in the dashboard related to authentication states and API format standardisation.

Summary of changes:

• Standardized API endpoints (Transactions, Webhooks, API settings) to follow the envelope format {success: true, data: {items, page, total, limit}}.
• Hardened user sign-out action by completely wiping out the pv_token cookie.
• Created an error boundary with interceptor to track 401 Unauthorized responses specifically originating from /dashboard/* endpoints. It automatically kicks out expired JWTs and routes users safely to the sign-in screen without silent API failures.