feat: update banners for credit based trial plan

[AdityaHegde]

Updates the banners to support the new credit based trial plan. Follows: https://www.notion.so/rilldata/Cloud-First-Burn-Down-Plan-343ba33c8f57813fb3abdc4e4bcaf042?source=copy_link#34aba33c8f578023b0fffeb698b52304

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[djbarnwal]

Developed the below feedback with Codex:

• [P1] Dismissing the welcome banner hides later low-credit warnings forever.
  web-admin/src/features/billing/issues/getMessageForTrialPlan.ts:105 and :110 both use buildDismissableForIssue, which stores id: issue.type at :150. Since welcome and low-credit are both BILLING_ISSUE_TYPE_ON_CREDIT_TRIAL, a forever dismissal from the welcome state makes isBannerDismissed return true later at web-common/src/components/banner/banner-dismiss.ts:16-17. Admins who dismiss “Welcome to Rill” will never see the 80% warning.

• [P1] The 50% and 90% credit flows are not implemented.
  getMessageForCreditsTrial only branches on onCreditTrial.lowCredit at web-admin/src/features/billing/issues/getMessageForTrialPlan.ts:101. The API shape only exposes creditAllocation and lowCredit (web-admin/src/client/gen/index.schemas.ts:187-191), so the UI cannot show the 50% blue weekly banner or the 90% red “almost out” banner with 4-hour/session dismissal.

Feel free to ignore them if they are not relevant to this PR.

[AdityaHegde]

Thanks @djbarnwal . Fixed the issue with low credits, good catch. We are not doing 50% & 90% right now, just 80%. Updated the doc to clarify.

[AdityaHegde]

Merging since the test is unrelated and on main.