fix: correct iOS Safari viewport height on sign request

[bdebyl]

📝 Summary

Fixes the external signing page being cut off at the bottom on iOS Safari.

ExternalApp.vue sizes html body #content — and, under max-width: 512px, #app-sidebar — to height: 100vh with position: fixed. iOS Safari resolves 100vh against the large viewport i.e. as though the browser chrome were hidden. The element therefore extends behind the bottom toolbar, and its lower edge cannot be scrolled into view / is inaccessible.

On the signing page that lower edge is the action bar, so a signer on an iPhone sees it clipped and cannot reliably tap it. Signing is effectively blocked on mobile Safari.

This pairs each 100vh with a following 100dvh. The dynamic viewport unit tracks the browser chrome as it shows and hides, which is the intended behaviour here.

Why it is paired with 100vh

The 100vh declaration is kept first, so a browser without dvh support simply drops the second declaration and behaves exactly as it does today. From my understanding, no @supports guard is needed and there is no regression risk for older clients.

Note: I did test on other browsers, but the before/after is the same for desktop.

🧪 How to test

1. Open signature request page on iOS with Safari or other mobile browser
2. The bottom <aside> allowing signature defining is no longer obscured by UI elements

🎨 UI / Front‑end changes

🏚️ Before	🏡 After

• Tested in multiple browsers (Chrome, Firefox, Safari) – optional but appreciated
• [n/a] Components, Unit (with vitest) and/or e2e (with Playwright) tests added - Required
• Accessibility verified (contrast, keyboard navigation, screen reader friendly) – if applicable
• [n/a] Design review approved – optional, link to feedback if available
• [n/a] Documentation updated (if applicable) – docs repository

✅ Checklist

• I have read and followed the contribution guide.