feat(view): Obsidian-style light/dark theme switch

[kzu]

Summary

• Add an Obsidian-style light/dark theme toggle to the right of the Tags header link.
• Theme the reader shell with CSS variables; persist preference in localStorage with FOUC-safe init and system preference fallback.

Test plan

• Open okf view output and confirm the switch appears right of Tags.
• Toggle light → dark and dark → light; verify shell, nav, content, and controls restyle correctly.
• Reload and confirm the chosen theme is restored.
• With no stored preference, confirm system prefers-color-scheme is respected on first load.
• dotnet test --filter FullyQualifiedName~BundleViewer

[kzu]

🧪 Details on Ubuntu 24.04.4 LTS

from retest v1.1.0 on .NET 10.0.9 with 💜 by @devlooped