feat: add congratulatory modal when pro trial begins

[devin-ai-integration]

Summary

Adds a congratulatory modal that appears when a user starts their Pro trial. Previously, nothing was shown to the user when their trial began. The modal follows the same pattern as the existing TrialExpiredModal.

Changes:

• Created TrialBeginModal component with "Welcome to Pro!" message and feature highlights
• Added modal to main app layout for global rendering
• Triggered modal when trial starts during onboarding (login.tsx)
• Triggered modal when trial starts from account settings (account.tsx)
• Added "Trial Begin" button to devtools panel for testing

Review & Testing Checklist for Human

• Test onboarding flow: Sign up as a new user eligible for trial and verify the modal appears after login completes. Confirm the modal doesn't block navigation to the next onboarding step.
• Test settings flow: As an existing user eligible for trial, click "Start Pro Trial" in Account settings and verify the modal appears after the trial starts.
• Test devtools: Open devtools panel and click "Trial Begin" button to verify the modal can be triggered manually.
• Verify modal styling: Check that the modal looks correct and matches the design system (compare with Trial Expired modal).

Notes

• The modal is triggered in onSuccess callbacks, so it should only appear after the trial has successfully started
• Modal uses the same zustand store pattern as TrialExpiredModal
• Requested by @ComputelessComputer
• Link to Devin run: https://app.devin.ai/sessions/9315be296d594ec6b578285fc4cd976a

[devin-ai-integration]

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