fix(settings): add fallback text to webauthn error mapping and clarify field names

[vpomerleau]

Because

• The error map only stored FTL keys, leaving callers without a single source of truth for fallbacks
• Field names (messageKeys, userMessageKey) were ambiguous

This pull request

• Renames messageKeys → ftlId and userMessageKey → ftlId on ErrorEntry and CategorizedWebAuthnError
• Adds fallbackText field with English strings matching the FTL file exactly
• Extracts UNEXPECTED_FALLBACK and UNEXPECTED_FTL_ID constants to eliminate duplication
• Updates tests with fallbackText assertions across all error categories

Issue that this pull request solves

Issue: FXA-12912

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[Copilot]

Pull request overview

This PR updates the passkeys WebAuthn error mapping in fxa-settings to make localization usage clearer and to provide a single source of truth for both Fluent IDs and English fallback strings.

Changes:

• Renames the error message identifier fields to ftlId (from messageKeys / userMessageKey) for clearer semantics.
• Adds fallbackText (English) alongside each FTL ID in the WebAuthn error map.
• Refactors unexpected-error defaults into shared constants and updates tests to include fallback-related assertions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
packages/fxa-settings/src/lib/passkeys/webauthn-errors.ts	Renames message key fields, adds per-operation fallbackText, and introduces shared unexpected-error constants.
packages/fxa-settings/src/lib/passkeys/webauthn-errors.test.ts	Updates assertions to use ftlId and adds coverage for fallbackText.

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