Add file-based logging foundation

[dfederm]

Summary

This adds a persistent Microsoft.Extensions.Logging foundation for JellyBox so diagnostics from sideloaded UWP/Xbox builds survive beyond debugger/console output. A custom rotating file ILoggerProvider writes to the app's ApplicationData.Current.LocalFolder\logs folder for Device Portal retrieval, is registered through DI, and is flushed during crash/suspend paths. Representative app, auth, navigation, library/search/item, and playback diagnostics now use injected ILogger<T> with CA1848-compliant [LoggerMessage] source-generated methods.

Why this is safe

• The sink is dependency-light: no new NuGet packages, app-local storage only, 4 MiB log rotation with 3 retained files, and non-blocking normal enqueue so logging does not stall app threads.
• Crash and suspend paths use a bounded synchronous flush so recent entries are persisted without an unbounded hang.
• Registration is additive: the existing console provider remains, and the concrete FileLoggerProvider singleton is the same instance used by the logging factory and flush path.
• Log messages capture breadcrumbs and failures without passwords or access tokens; verbose playback traces are Debug level and remain silent with the default Information minimum level.
• Unobserved task exceptions are logged and marked observed so they do not terminate the process after capture.

Validation

Test / build	Result
msbuild JellyBox.sln -t:Build -p:Configuration=Debug -p:Platform=x64	Passed locally; build succeeded with 0 warnings and 0 errors.
Unit tests	Not run; this repository has no test project.

Out of scope

• This is the logging foundation only; full call-site coverage, log-level UI, share-logs UX, and crash-reporting/telemetry service integration remain future work.
• Heads-up: open PR Fix shell search suggestion pick after dismissing OSK with B #101 also touches App.xaml.cs and AppServices.cs; this PR's edits there are limited to additive logging registration/lifecycle hooks, so a small rebase may be needed.

Linked issues / todos

• Related: Work to do before shipping MVP #1 (MVP checklist Logging item)