Move dependency cache to Cache/SystemTools; add startup dependency checks and cleanup; simplify NextClass component

[Programmer-MrWang]

Motivation

• Move plugin-downloaded native/model dependencies out of Extensions/SystemTools into the global cache area so multiple plugins/installs share a predictable Cache/SystemTools path.
• Prevent runtime errors and lockouts when optional features (FFmpeg, face-recognition) are enabled but required files are missing by disabling those features at startup and cleaning up stale references.
• Simplify the 下节课是 component UI to a single-line layout and allow independent toggles for displaying the time range and teacher name.

Description

• Change dependency root resolution in Shared/DependencyPaths.cs to use the app cache path ..\..\Cache\SystemTools and add helpers HasFfmpegDependency(), HasFaceRecognitionDependencies(), and GetFaceRecognitionRequiredPaths().
• Add startup dependency enforcement in Plugin.cs via EnsureOptionalDependencyState() that will disable EnableFfmpegFeatures or EnableFaceRecognition if required files/folders are missing, and set flags used to emit startup warnings.
• Add Shared/FaceRecognitionCredentialCleanup.cs to remove systemtools.authProviders.faceRecognition entries from ClassIsland management credential JSONs when face-recognition dependencies are not present to avoid blocking users.
• Update settings and download logic to check the new dependency helpers (SettingsPage/SystemToolsSettingsViewModel.cs and SettingsPage/SystemToolsSettingsPage.axaml.cs) and adjust user-facing messages to require models + runtimes for face recognition.
• Replace the NextClassDisplay component layout and settings: remove the embedded progress/lesson control, display Prefix / Subject / TimeRange / Teacher on a single centered line, add ShowTimeRange setting, and make the teacher field show only the name (no label). Files changed include Controls/Components/NextClassDisplayComponent.*, Models/ComponentSettings/NextClassDisplaySettings.cs, and its settings control XAML.
• Ensure FFmpeg-dependent action registration and the utility menu consider the effective EnableFfmpegFeatures state after checks by gating menu/action registration accordingly.

Testing

• Ran repository checks: git diff --check and git status --short with no code style errors reported.
• Performed static code inspections and searches (multiple rg/sed) to verify updated call sites and settings usage succeeded.
• Attempted to run dotnet build SystemTools.csproj -p:EnableWindowsTargeting=true but the environment does not have the dotnet SDK installed so a full build could not be executed in this session.
• Committed the code changes after local verification of modified files and basic runtime-path logic via code review.

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[chatgpt-codex-connector]

Migrate existing dependency downloads before switching cache root

GetDependencyRoot() now only resolves ../../Cache/SystemTools, but the previous DependencyPaths implementation stored ffmpeg.exe and the face-recognition pack under the old plugin-root path (../../Extensions/SystemTools, i.e. the plugin directory itself). On an in-place upgrade, EnsureOptionalDependencyState() will therefore see already-downloaded dependencies as missing on the first launch, disable EnableFfmpegFeatures/EnableFaceRecognition, and even run the face-recognition credential cleanup. Users who already had these optional features working will lose them until they manually re-download and re-enable everything, so this path switch needs a migration or fallback to the old location.

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