feat: add primary-cta data-testid to default Button component

[wagmiiii]

Closes #412

Summary

Added stable data-testid attributes to primary CTAs for Playwright and analytics without coupling them to text content or classes.

What changed

• Created a central constants file at src/config/testIds.ts exporting TEST_IDS.PRIMARY_CTA.
• Updated the Button component to automatically apply this test ID when variant="primary" (the default variant) and no overriding data-testid is supplied.
• Updated docs/COMPONENTS.md to formally document this new accessibility/testing capability for the Button primitive.

Acceptance Criteria Checklist

• The change matches the summary above.
• No regression in the existing test suite.
• The change is documented where it is observable (docs/COMPONENTS.md).
• Lint, type-check, and tests all pass locally.
• PR description references this issue with Closes #.

Security Note

This change is purely presentational markup used for E2E testing and analytics hooks. No user-input paths, validation logic, or server-side interactions were altered.

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