Make long running steps cancellable and keep the queue from stalling the UI

[Kentarohakase]

Stacked on #2. Phase 2 of the improvement plan.

Fixed

• Adding files left the window half usable. LoadInputFilesAsync guarded on IsBusy but never raised it. While a dropped folder was analyzed, Cancel was disabled, Start was enabled and further drops were accepted. It now runs as a busy phase with its own cancellation source, so the analysis can actually be stopped. PrepareRetryItemsAsync follows the same shape.
• The processed preview render could not be stopped. It is a full FFmpeg pass over the loudest section and ran with CancellationToken.None. It now runs as a busy phase too — which is what makes Cancel reach it at all — and reports a cancellation as a normal outcome.
• A cancelled analysis would have crashed the app. MainWindow_OnDrop is async void with no guard, so an escaping OperationCanceledException would have reached the dispatcher handler. Both entry points handle it themselves.
• The startup tasks were fire and forget. PrepareYtDlpAsync and CheckForAppUpdateAsync were started with _ = …, never tracked and never cancelled. They are held in fields now, cancelled on shutdown, and check the token before writing view model state again. Their catch-all no longer hides real defects: a cancellation stays silent, anything else is logged.

Changed

• The queue rebuilt everything on every progress tick. OnBatchItemPropertyChanged called UpdateBatchSummary() and RaiseCommandStates() for every property of every item, including Progress, which is written on each FFmpeg tick. RaiseCommandStates makes WPF re-evaluate 24 predicates synchronously on the UI thread — several do file system calls, three allocate an array over the whole queue, and two build a complete filter plan. The refresh is now limited to the properties that actually feed the summary counts and the command predicates (Status, ComparisonReport, HasComparisonWarnings, OutputPath).
• The tool source was a German literal. Benutzer-Tools, App-Ordner and Tools-Ordner were built in ToolDiscoveryService and handed straight to the app bar, so an English user read German labels in an otherwise translated interface. ToolLocation now carries a resource key that ToolStatus.DisplayText resolves.
• A broken tool was probed again on every call. Only successful statuses were cached. With the bounded probe from Keep failing commands, slow tools and untrusted update links from breaking the session #2 that costs the full 20 seconds each time. Failures are cached too now, but only for 30 seconds, so a tool installed while the app runs is still picked up.

Two deviations from the plan

• The semaphore dispose race does not exist. The plan listed a fix for AnalyzeBatchItemsAsync disposing its SemaphoreSlim while releasers were still running. Task.WhenAll waits for all tasks to complete, not just until the first one fails, and every task releases in its own finally before completing — so the using never disposes early. No fix was needed and none was made.
• Dispose cancels the startup tasks but does not await them. They resume on the dispatcher, so blocking that thread while it has to run their continuations would deadlock. Cancelling plus a token check before each state write achieves the same end without the deadlock.

Verification

dotnet format .\AudioQualityEnhancer.slnx --verify-no-changes   -> clean
dotnet build  .\AudioQualityEnhancer.slnx -c Release            -> 0 warnings, 0 errors
dotnet test   .\AudioQualityEnhancer.slnx -c Release            -> 204 passed, 0 failed

The UI behaviour is not covered by tests and needs a manual pass: drop a folder with many files and check Cancel works; start a batch export and check the window stays responsive; switch the language to English and check the tool source in the app bar.