Update Zed trial copy for the $5 GPT Luna offer (#63969)

## Summary

Updates Zed trial copy for
[CLO-1373](https://linear.app/zed-industries/issue/CLO-1373/update-zed-to-show-the-dollar5-gpt-luna-trial).

- Show $5 of GPT Luna and unlimited edit predictions across pre-sign-in,
Free-plan, and active-trial onboarding.
- Clarify that the 14 days begin when the trial starts, rather than when
the welcome panel is viewed, and retain no-credit-card messaging.
- Update hosted-model trial descriptions and use "Start Free Trial" for
onboarding and AI configuration trial CTAs.
- Document GPT Luna-only trial hosted-model access and the shared
Zed/Delta $5 trial balance.

This is a copy-only change. Trial eligibility, subscription creation,
trial timing, billing, model-access enforcement, button actions, URLs,
and telemetry are unchanged. Paid, Student, Business, and VIP plan
descriptions are unchanged. This does not implement the offer on the
backend.

## Verification

- Rust formatting and `git diff --check` passed.
- Prettier check passed for the pricing documentation.
- Source checks confirmed the offer wording and unchanged callbacks,
URLs, telemetry, and other plan definitions.
- Katie built and launched Zed with `cargo run -p zed --bin zed`, then
reviewed Component Preview and requested the final CTA wording.
Formatting and diff checks were rerun after that wording adjustment; the
final adjustment has not been rebuilt.
- Confirmed the branch merges cleanly with the latest `main`.

Release Notes:

- Improved trial descriptions to show $5 of GPT Luna and unlimited edit
predictions for 14 days from trial start.

docs/src/account/plans-and-pricing.md

@@ -27,7 +27,7 @@ Zed is free to use. You can configure AI agents with your own API keys via [Use
 
 ### Zed Pro {#pro}
 
-Zed Pro includes access to all hosted AI models and Edit Predictions. The plan includes $5 of monthly token credit; usage beyond that is billed at the rates listed on [Zed-Hosted Models](./zed-hosted-models.md). A trial of Zed Pro includes $20 of credit, usable for 14 days.
+Zed Pro includes access to all hosted AI models and Edit Predictions. The plan includes $5 of monthly token credit; usage beyond that is billed at the rates listed on [Zed-Hosted Models](./zed-hosted-models.md). A [trial of Zed Pro](#trials) includes $5 of GPT Luna and unlimited Edit Predictions for 14 days from when you start the trial.
 
 For details on billing and payment, see [Individual Billing](./billing.md).
 
@@ -65,4 +65,6 @@ On Zed Business, administrators set a pre-tax org-wide spend limit from the Data
 
 ### Trials {#trials}
 
-Trials automatically convert to Zed Free when they end. Trials do not include access to Anthropic's Opus models. No cancellation is needed to prevent conversion to Zed Pro.
+Trials include $5 of GPT Luna and unlimited Edit Predictions for 14 days from when you start the trial. Trial hosted-model access is limited to GPT Luna. Zed and Delta share the same $5 trial balance. No credit card is required.
+
+Trials automatically convert to Zed Free when they end. No cancellation is needed to prevent conversion to Zed Pro.