fix: use Viewer for public whiteboard shares on NC29

[hweihwang]

Summary

Public share links to whiteboard files on Nextcloud 29 render inside the legacy files_sharing public preview container (#preview/#imgframe), which has its own header/footer and constrained layout. This causes the whiteboard to look boxed and misaligned compared to NC32+.
This change opens public whiteboard shares via the Nextcloud Viewer modal for a consistent fullscreen experience.

Root Cause

On NC29, the whiteboard embeds inside #imgframe which has constrained dimensions (height: 75%, width: 80%, padding: 32px). NC32+ rewrote files_sharing to automatically trigger Viewer for file shares, but NC29's public.js only auto-opens Viewer for image/video/audio - not custom MIME types like whiteboard.
A CSS-only fix doesn't work because hiding the header breaks the height: 100% cascade throughout the legacy template structure.

Solution

• Register the whiteboard Viewer handler on public share pages
• Call viewerApi.openWith('whiteboard', ...) to open in a fullscreen Viewer modal
• Fallback to embedded rendering if Viewer is unavailable
• Add scoped CSS to hide header/footer and adjust Viewer styling
• Skip script injection on password prompt pages

Scope

• Only affects public shares of whiteboard files
• Non-whiteboard public shares and authenticated Viewer behavior unchanged