fix: 2FA Server Key setup — error toast and QR code fails to load on iOS

[Copilot]

On iOS, navigating to "Set up 2FA for Server Key" threw Cannot read properties of undefined (reading 'model'), left the screen permanently spinning, and sometimes rendered only the manual key with no QR code.

Root causes

• SigningServer.ts — silent error swallowing (13 methods): The two-branch catch pattern skipped rethrowing when err had neither .response nor .code (network timeout, SSL failure on iOS). res stayed undefined; the next res.data access produced a secondary TypeError whose property name varied by path ("model", "data", etc.).

  // Before — silently falls through when neither branch matches
  } catch (err) {
    if (err.response) throw new Error(err.response.data.err);
    if (err.code)     throw new Error(err.code);
  }
  
  // After — always throws a meaningful message
  } catch (err) {
    throw new Error(err.response?.data?.err || err.code || err.message || err.toString());
  }

• SetupSigningServer.tsx — unsafe access + wrong guard: setupData.verification.verifier threw on malformed server responses; validationKey === '' didn't catch undefined/null, allowing QR render with a falsy key.

• No error recovery UX: A failed registration left the screen stuck in a permanent spinner with no path to retry.

• KeeperQRCode — no render-error escape hatch: react-native-qrcode-svg accepts onError; without it, generation failures in the component's useMemo were rethrown uncaught.

Changes

• SigningServer.ts — applied safe rethrow to all 13 axios catch blocks.
• KeeperQRCode.tsx — added optional onError?: (error: Error) => void prop, forwarded directly to <QRCode>.
• SetupSigningServer.tsx
  • Guard: validationKey === '' → !validationKey
  • Safe access: setupData?.verification?.verifier; explicit registrationError state set when verifier is missing
  • Error state: replaces spinner with a Retry button on failure; onError on <KeeperQRCode> surfaces render failures as a toast
• SetupAdditionalServerKey.tsx — same !validationKey guard for parity.