Add Renovate config

[danielchalmers]

Adds a Renovate configuration (.github/renovate.json) so dependency updates are automated.

Notes

• Extends config:recommended; no schedule, so update PRs open as soon as new versions are published.
• Non-major NuGet updates are grouped into a single PR to keep noise down; GitHub Actions updates are grouped likewise.
• Major updates of Microsoft.AspNetCore.* are disabled — those packages are pinned per target framework (net9.0 → 9.0.x, net10.0 → 10.0.x), so a major bump should only happen alongside a TFM upgrade.
• Major MudBlazor updates require approval from the dependency dashboard, since they typically involve breaking API work (like the v9 upgrade in Upgrade to .NET 10 and MudBlazor v9 #39).