Upgrade SanpetePantry to .NET 9 and update all packages to latest versions

[Copilot]

This PR upgrades the SanpetePantry project from .NET 8 to .NET 9 and updates all NuGet packages to their latest versions for improved performance, security, and access to the latest framework features.

Changes Made

Framework Upgrade

• Updated target framework from net8.0 to net9.0 in the project file
• Updated Dockerfile base images from .NET 8 to .NET 9:
  • mcr.microsoft.com/dotnet/aspnet:8.0 → mcr.microsoft.com/dotnet/aspnet:9.0
  • mcr.microsoft.com/dotnet/sdk:8.0 → mcr.microsoft.com/dotnet/sdk:9.0

Package Updates

Updated all packages to their latest versions:

• Microsoft packages: 8.0.0 → 9.0.8
  • Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore
  • Microsoft.AspNetCore.Identity.EntityFrameworkCore
  • Microsoft.AspNetCore.Identity.UI
  • Microsoft.EntityFrameworkCore.Sqlite
  • Microsoft.EntityFrameworkCore.Tools
• mailkit: 3.2.0 → 4.13.0
• Microsoft.VisualStudio.Azure.Containers.Tools.Targets: 1.19.5 → 1.22.1
• FluentEmail packages: Kept at 3.0.2 (latest available)

Code Quality Improvements

• Fixed duplicate using directive warning in Routes.razor by removing redundant @using Shared directive (already imported globally in _Imports.razor)
• Project now builds with zero warnings (previously had 1 warning)

Testing

• ✅ Project builds successfully with .NET 9
• ✅ Application runs correctly and starts without errors
• ✅ All packages are at their latest compatible versions
• ✅ No breaking changes detected in the upgrade

Benefits

• Performance: Access to .NET 9 runtime optimizations and improvements
• Security: Latest security patches and updates in all dependencies
• Maintainability: Staying current with Microsoft's supported .NET versions
• Future-proofing: Positioned for upcoming .NET ecosystem improvements

Fixes #20.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• aka.ms
  • Triggering command: curl -I -sSL --retry 5 --retry-delay 2 --connect-timeout 15 REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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