chore(onboarding): align founder note and CTA with landing page copy

[quiet-node]

Description

Aligns the founder note and email CTA on the onboarding subscribe screen (and the Settings About tab) with the landing-page wording, so all three surfaces read in one consistent voice.

Key changes

• Rework the founder note: lead with "Hey, I'm Logan, the founder here.", add the landing page's "Thuki is early and I'm building it around how people really use it" hook, and drop the duplicated "I'd love to" phrasing.
• Standardize the sign-off to "I'll reach out personally. Thanks!" (matches the landing page).
• Rename the onboarding CTA from "Help shape what's next for Thuki" to "Count me in" (landing-page parity, lower friction).
• Mirror the same note voice in the Settings About tab, keeping its "shape what's next" phrasing since no roadmap list is shown there.
• Update the affected SubscribeStep tests to the new copy.

Testing

• bun run test:all:coverage (100% gate holds)
• bun run validate-build (clean)