fix(mobile): explicitly inject traceparent for mobile↔API correlation

[davidortinau]

Summary

Explicitly inject W3C traceparent into outbound HttpClient requests from mobile so the API can correlate its spans against ours.

Why

PR #172 added ApiActivityHandler which creates an Activity per request (so App Insights dependencies now carry operation_Id). But the correlation join still returned zero rows:

requests
| where cloud_RoleName endswith "SentenceStudio.Api"
| join kind=inner (dependencies | where cloud_RoleName == "SentenceStudio.Mobile") on operation_Id

Evidence after #172:

• Q1 (mobile deps with operation_Id): 44 rows ✅
• Q2 (correlation join): 0 rows ❌

Diagnosis: HttpClient's built-in DiagnosticsHandler only injects traceparent automatically when an ActivityListener is attached to "System.Net.Http". On MAUI that listener never attaches because OpenTelemetry's TelemetryHostedService — which materializes the TracerProvider and its listeners — depends on IHostedService, and MauiApp doesn't run hosted services. Tracked in #171.

What changed

In ApiActivityHandler.SendAsync, after the activity null-check and tag-setting, call DistributedContextPropagator.Current.Inject(...) on the request headers. Guarded by !request.Headers.Contains("traceparent") so deliberate upstream callers and resilience retries don't produce duplicate headers.

Why this is the right scope

• Minimal surface: 26 lines in one file, no new dependencies.
• Uses DistributedContextPropagator.Current (W3C by default in .NET 8+), not a captured instance — safe if runtime defaults change.
• Doesn't affect the null-activity early-return path.
• Complements rather than replaces the framework fix tracked in MAUI OTel: force TracerProvider materialization since IHostedService doesn't run #171; MAUI OTel: force TracerProvider materialization since IHostedService doesn't run #171 is now lower priority.

Verification

1. Build Mac Catalyst Debug + Release ✅ (compile clean; pre-existing universal-bundle plist merge issue is unrelated).
2. Publish to DX24 via iOS Release.
3. Re-run Q2 correlation join ~2 min after generating traffic. Expected: > 0 rows.

Refs: #165 #166 #172 #171