[ecash-parse] Compact notification amounts; open app on push tap

Summary: Format XEC and token amounts in push/in-app notification copy with k/M/B/T for readability (e.g. Agora sales, receives). Tapping a push opens/focuses Cashtab: web notificationclick + foreground onclick, Android OPEN_CASHTAB clickAction + intent-filter.

Test Plan: pnpm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D20294