chore: seedless error with causes details TO-168

[ieow]

Explanation

Adds a new generic SeedlessOnboardingError class to handle errors in the SeedlessOnboardingController that don't fit the more specific PasswordSyncError or RecoveryError classes.
Changes

• Added SeedlessOnboardingError class with:
• details property for additional debugging context beyond the error message
• cause property to preserve the original error chain
• toJSON() method for proper serialization in logging/error tracking systems

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Introduces a generic SeedlessOnboardingError with optional details, cause propagation (Error/string/object), and toJSON() serialization for structured logging.

• New SeedlessOnboardingError class in errors.ts; comprehensive tests added in errors.test.ts
• SeedlessOnboardingController.authenticate now throws SeedlessOnboardingError with cause on failure
• Public API updated to export SeedlessOnboardingError from index.ts

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[cursor]

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