Fix unhandled rejection when cancelling MFA setup after enable

[posthog-eu]

Description of Changes

Why: A user who finished two-factor setup in onboarding tripped a silent unhandled promise rejection that error tracking reports as Error: {"error":"MFA already enabled"}. The failure is benign for the user — setup completes and no toast shows — but it is noise in error tracking and a latent floating-rejection pattern in the desktop client.

Problem

• The MFA onboarding slide cleanup cancels the pending setup on unmount when setupCompleteRef.current is false (MFASetupSlide.tsx).
• That ref was synced from state in a separate effect, but handleEnableMfa flips the state and calls onMfaSetupComplete?.() in the same tick.
• If the parent unmounts the slide before the sync effect commits, the cleanup cancels a setup where MFA is already enabled; the backend answers HTTP 409.
• The void discarded the promise, so nothing caught the rejection and autocapture picked it up. On desktop the body is wrapped into an Error by TauriHttpClient.createError.

Changes

• Set setupCompleteRef.current = true directly when enable succeeds, so the cleanup skips the cancel without waiting for a follow-up effect to commit.
• Settle the cancel promise with .catch(), so a 409 is a no-op rather than a floating rejection. This mirrors the try/catch already used in AccountSection.tsx.
• Remove the now-redundant effect that mirrored state into the ref.

Treating a 409 as a no-op is correct: once MFA is enabled there is no pending secret left to clear.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

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• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
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Translations (if applicable)

• I ran scripts/counter_translation.py

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have run task check to verify linters, typechecks, and tests pass
• I have tested my changes locally. Refer to the Testing Guide for more details.

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