Add web UI dashboard with health status, #90

[paulirwin]

Closes #90 (first sub-issue of #68).

Adds a web UI at the emulator's root URL showing basic health status, plus the health-check infrastructure behind it.

What's here

The dashboard — served at /, showing overall status, version, environment, uptime, the indexes directory, and each check's result. It refreshes itself every 15 seconds over the Blazor circuit and has a manual refresh button.

Blazor, per the issue, so there's no separate JS framework to build or keep in sync — the whole UI lives in the same assembly. It's written from scratch rather than from the Blazor template: the template brings bootstrap, a sample stylesheet and the default error UI along for what is a handful of cards. The stylesheet is ~340 lines of hand-written CSS with tokens for light and dark, following the developer's OS theme.

Two health checks, surfaced on the dashboard and at /health:

• index-storage — can the indexes directory actually be written to? This is the one worth having. A read-only Docker volume, or a dotnet tool launched from a directory the user can't write to, currently surfaces as a 500 from whichever indexing request happens to run. Writability is probed with a real write, because no file attribute distinguishes a read-only bind mount from a full disk from a POSIX mode bit.
• index-definitions — does every definition on disk still deserialize? These are JSON files users are told about and mount as a volume, so hand-editing one is expected. A malformed file currently takes out an unrelated listing request. Reports Degraded rather than Unhealthy, since every index that does parse is still being served.

/health is kept separate from /servicestats, which Aspire already probes — that route imitates Azure's API surface and answers 200 as long as the process is up.

Two accommodations for owning /

Both are the kind of change that works locally and breaks the API surface elsewhere, so the integration tests assert the API side in the same file.

• OData's service document gave up the root. AddRouteComponents("", model) puts MetadataController.GetServiceDocument on /, which made the dashboard's @page "/" an ambiguous match — the request failed with AmbiguousMatchException rather than either endpoint winning. The service document is the one to drop: Azure AI Search doesn't serve one (its root 404s) and the Azure SDK never requests it. /$metadata is a separate action on the same controller and is unaffected — there's a test for that.
• UseAntiforgery is now in the pipeline, required by MapRazorComponents. It ignores requests that carry no token, so unauthenticated SDK writes are unaffected — ApiWrites_AreNotBlockedByAntiforgery drives a real index create/read/delete through the SDK to pin it.

Testing

• 8 unit tests over the health checks, against real directories (the conditions they exist to catch are filesystem states; a mocked filesystem would only test the mock's opinion of a failed write). Covers healthy, not-yet-created, unreachable, non-writable, no-probe-file-left-behind, and the malformed-definition path.
• 5 integration tests against the containerized emulator: the dashboard renders with prerendered status in the markup, the stylesheet serves, /health answers, $metadata survives, and SDK writes aren't blocked.
• Full suite green: 981 unit + 225 integration + 7 Aspire, no regressions.

Verified by hand against a running instance as well — root, /health, /servicestats, /indexes, /$metadata, index create, document index, and delete all behave as before.

README gains a "Web UI" section covering the dashboard, /health, and the service-document note.

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