feat: add an Aspire sample host for local testing and as a setup demo

[andregoepel]

Adds a runnable example under samples/ so the foundation can be exercised end-to-end and so the host-side wiring is demonstrated in-repo (the real consumer lives in a separate repo).

What

Project	Role
samples/AndreGoepel.AppFoundation.Sample	Minimal Blazor Server host consuming the packages — the reference Program.cs, App.razor, Routes.razor, _Imports.razor, a / landing page, and wwwroot/app.css. Wires AddAppFoundation / UseAppFoundation, the router's additional assemblies, and MapAdditionalIdentityEndpoints.
samples/AndreGoepel.AppFoundation.AppHost	.NET Aspire orchestrator — starts PostgreSQL (with a data volume) and the web app, wiring the appfoundation-database connection string and WaitFor-ing the DB.

Run it:

dotnet run --project samples/AndreGoepel.AppFoundation.AppHost

Open the Aspire dashboard, start the web resource; the first visit funnels to /Setup (create the admin), then sign in and explore the dashboard + Administration. A README "Try it locally" section documents this, and .aspire/settings.json now points at the in-repo AppHost.

Notes / decisions

• In Development everything runs with zero extra config — AutoCreate.All, no key-ring cert required (the fail-closed check is non-Dev only), forwarded-headers trust-all. So the demo just works against the Aspire-provided Postgres.
• AppHost opts out of CPM and lock files. The Aspire.AppHost.Sdk injects RID-specific orchestration/dashboard packages, so a committed lock file would be win-x64/linux-x64 dependent and break CI's --locked-mode restore across OSes. The sample keeps a portable lock file; both projects are IsPackable=false, so nothing new is published.
• Self-contained inline SVG logo so the sample ships no binary asset.

Verification

• dotnet build -c Release (full solution incl. both new projects) — 0 errors.
• dotnet restore --locked-mode — clean (AppHost correctly excluded).
• dotnet test -c Release — 80/80 (unchanged).
• The wiring was checked against the AndreGoepel.Marten.Identity.Blazor package's own README and public API (MapAdditionalIdentityEndpoints, RedirectToLogin, LoginLayout, Aspire 13.x AddPostgres/AddDatabase/AddProject<T>/WithReference/WaitFor).

⚠️ Not runtime-verified here: launching the full Aspire stack needs a container runtime and is a long-running orchestration, so I verified build + CI-parity + API conformance rather than a live dotnet run. Worth one manual dotnet run on your side to confirm the dashboard/web come up and /Setup flows.

Branched off main, so it's independent of the open feature PRs.

[andregoepel]

Rebased this branch onto feat/configurable-login-features (#67) and retargeted the PR base to it, so the sample now demonstrates the Administration → Login Features admin page too (it lives in #67, which wasn't on main). The sample lock file was synced to AndreGoepel.Marten.Identity.Blazor 1.2.0-preview1 to match.

Merge order: #67 first, then this. Once #67 lands on main I'll rebase this onto main and retarget the base back to main.