Add lightweight page-load performance instrumentation

[ledpup]

Motivation

• Provide lightweight, low-cost diagnostics to measure page-load and chart-render performance without adding a full telemetry provider.
• Make it easier to triage slow paths (reference data, dashboard anomaly/stripe work, and chart dataset preparation).
• Capture API client timings (including cache state and record counts) to spot expensive or duplicated requests.

Description

• Added a reusable C# timing scope PerformanceLogScope at ClimateExplorer.Web.Client/Infrastructure/PerformanceLogScope.cs that emits PerfStart/PerfEnd log lines with elapsed milliseconds and contextual fields.
• Added a small browser helper window.climateExplorerPerformance in ClimateExplorer.Web/wwwroot/js/performance-instrumentation.js and load it from the app shell (ClimateExplorer.Web/App.razor) to emit ClimateExplorer: Performance marks/measures to DevTools and the Performance timeline.
• Instrumented key client lifecycle and UI paths with both server logs and browser marks: Index page lifecycle and reference-data load (Index.razor.cs), LocationDashboard deferred data loading (LocationDashboard.razor.cs), and chart build/render flows in ChartView (ChartView.razor.cs).
• Enhanced the API client (ClimateExplorer.WebApiClient/Services/DataService.cs) to log PostDataSet timings and to centralise GET timings via GetFromJsonWithTiming, including cache state and record counts; added docs/performance/page-load-instrumentation.md describing instrumentation and how to compare cold/warm loads.

Testing

• Ran git diff --check to validate whitespace and basic diffs, which succeeded.
• Attempted dotnet build ClimateExplorer.sln, but the build could not be executed in this environment because dotnet is not installed (automated build unavailable here).
• No runtime/integration tests were run in this container due to missing .NET SDK; manual verification is documented in docs/performance/page-load-instrumentation.md for local testing with the full dev setup.

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Codex Task