task(passkeys): Add WebAuthn error categorization utility

[vpomerleau]

Because

• Raw DOMExceptions from createCredential() / getCredential() must be translated into semantic, UI-safe categories before they reach components
• Error handling strategy (Sentry routing, user messaging) differs by both error type and ceremony context (registration vs. authentication)
• WebAuthn browser errors are explicitly out of scope for auth-errors and must not use the errno-based registry (see FXA-12910 for server errors)

This pull request

• Adds categorizeWebAuthnError(error, operation) — maps DOMException names and TypeError to one of three categories (UserAction, DevicePlatform, Unexpected), returns an operation-specific FTL key and logToSentry flag; never throws
• Adds handleWebAuthnError(error, operation, captureException) — single call site that categorizes the error and conditionally fires Sentry
• Covers all WebAuthn Level 3 error types including DataError and EncodingError (thrown by parseCreationOptionsFromJSON / parseRequestOptionsFromJSON for malformed options)
• Adds FTL strings for all 15 error/operation combinations

Issue that this pull request solves

Closes: FXA-12912

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

Screenshots (Optional)

Please attach the screenshots of the changes made in case of change in user interface.

Other information (Optional)

Based on FXA-12911, only latest commit is relevant for this PR. Will rebase once previous PR is finalized and merged.

[vbudhram]

Reviewed c9935ea since its built on #20088. This is only adding error file with ftl so pretty low risk and can fix issues in upcoming PRs.