Stats: subscription stats UX issues

See design P2s, internal reference:

- i1: p1HpG7-v7v-p2
- i2: p1HpG7-vch-p2

#### Small UX issues around subscription stats:

- #40537

- [x] Emails/Subscribers tab consolidation (possibly done already -> review placements and copy)

- [x] #40538

- #40608
- [x] Remove redundant data (Today, 30 days ago, 60 days ago and 90 days ago) https://github.com/Automattic/wp-calypso/pull/97670
- [ ] Introduce Key Metrics at the top to give a quick overview of newsletter performance.

- #40646
- #40648
- https://github.com/Automattic/wp-calypso/issues/87174
- [ ] Paid subs+free subs graph; free subs looks flat when I have paid subs and enabling/disabling stats is clunky. See improved design (p1HpG7-vch-p2):

- https://github.com/Automattic/jetpack/issues/960

- [ ] Explore showing "subscribed via" -source; we track this already for ourselves but might require adding a database for customers.

- https://github.com/Automattic/wp-calypso/pull/97554

#### Single email issue view

- [ ] #40659

- [ ] Explore showing 24-hour chart!

- https://github.com/Automattic/jetpack/issues/40724

- https://github.com/Automattic/wp-calypso/pull/97817

Start by reviewing the linked design references, related issues, and pull requests, then identify which unchecked subscription-stats or single-email item is still in scope. Because the issue is an umbrella for several exploratory UX changes and names no files or tests, completion requires selecting a specific item and agreeing on its implementation and validation criteria.

Written by the indexing model from the issue text.