Migrate PDFKeeper to .NET 10 (LTS)

[rffrasca]

Description
This migration provides long‑term platform stability, modern API surface, improved performance, and access to current WinForms features required for future MVVM tightening.

Why This Is Needed

• .NET 10 is the latest LTS release with support through Nov 2028.
• Enables modernization work blocked on .NET Framework (e.g., WinForms command binding).
• Ensures compatibility with updated dependencies.
• Reduces technical debt and aligns with Microsoft’s supported platform lifecycle.

Scope

• Upgrade to Visual Studio 2026 to be able to target .NET 10 in the IDE.
• Update project targets to net10.0-windows.
• Update dependencies to versions compatible with .NET 10.
• Validate runtime behavior across all supported database platforms.
• Remove obsolete .NET Framework compatibility code.
• Update documentation to reflect new platform requirements.

Notes
This work must be completed before any other milestone issues that depend on (.NET 10) features or require removal of legacy framework constraints.