Fix nullability warnings in SDMetaUI

[Copilot]

• Fix Models/FilteredList.cs: make modelFilter and filter fields nullable, update property types and RunFilter()
• Fix Models/ImageFileViewModel.cs: make SubItems nullable
• Fix Models/ImageFileViewModelBuilder.cs: handle nullable FullPromptHash
• Fix Models/GalleryViewModel.cs: initialize Rows, make ModelFilter/Filter nullable, fix RemoveFile(), fix MovePrevious()/MoveNext()
• Fix Models/GroupedByPromptList.cs: change nullable fields to non-nullable with initialization
• Fix Shared/FilePanel.razor.cs: fix parameter default, nullable fields, OnParametersSetAsync() — clear all display fields when realFile == null to avoid stale metadata
• Fix Shared/FilePanel.razor: fix nullable promptLines in foreach
• Fix Shared/FullScreenView.razor.cs: fix parameter default, hxModal null handling
• Fix Shared/FullScreenView.razor: fix hxModal.HideAsync() call
• Fix Shared/Thumbnail.razor.cs: fix File parameter default, make Text nullable
• Fix Pages/Index.razor.cs: fix event handler signatures, nullable fields, null checks
• Fix Pages/Index.razor: fix nullable onInputDebounced and fullScreenView usages
• Fix Services/FileSystemObserver.cs: fix null passed to event invocation
• Fix Pages/Settings.razor: make dbPath nullable
• Fix RetryingFileLoader.cs: use null! for return
• Verify build has no CS8 warnings
• All 9 existing tests pass

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[Copilot]

Pull request overview

This PR updates SDMetaUI (and a few SDMeta core interfaces/DTOs) to be nullable-aware under <Nullable>enable</Nullable>, aiming to eliminate CS8xxx nullability warnings by tightening annotations, adding guards, and adjusting a couple of APIs.

Changes:

• Annotates component parameters/backing fields and view-model properties with nullable reference types and adds null-safe dereferences/guards.
• Adjusts file-system and loader APIs/events to align with nullable patterns (e.g., nullable GetImageFile result, event sender nullability).
• Refactors model-filter handling to use a shared AllModels sentinel and makes query DTOs nullable-friendly records.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
SDMetaUI/Shared/Thumbnail.razor.cs	Makes File required and allows nullable Text.
SDMetaUI/Shared/FullScreenView.razor.cs	Marks selectedFile required; uses null-forgiving on modal ref.
SDMetaUI/Shared/FullScreenView.razor	Uses null-forgiving on hxModal when hiding.
SDMetaUI/Shared/FilePanel.razor.cs	Makes display fields nullable and adjusts prompt assignment.
SDMetaUI/Shared/FilePanel.razor	Null-forgiving on promptLines enumeration under HasPrompt.
SDMetaUI/Services/FileSystemObserver.cs	Changes FileSystemChanged event type and adjusts Reset() invocation args.
SDMetaUI/RetryingFileLoader.cs	Updates loader return type to ImageFile?.
SDMetaUI/Pages/Settings.razor	Makes dbPath nullable.
SDMetaUI/Pages/Index.razor.cs	Makes several fields nullable, adjusts event signatures, adds null-guards, and simplifies InvokeAsync.
SDMetaUI/Pages/Index.razor	Uses null-forgiving for onInputDebounced and fullScreenView call.
SDMetaUI/Pages/Error.cshtml.cs	Removes primary constructor from ErrorModel.
SDMetaUI/Models/ModelSummaryViewModel.cs	Introduces AllModels sentinel and refactors constructors.
SDMetaUI/Models/ImageFileViewModelBuilder.cs	Ensures non-null FullPromptHash passed to VM constructor.
SDMetaUI/Models/ImageFileViewModel.cs	Makes SubItems nullable.
SDMetaUI/Models/GroupedByPromptList.cs	Initializes internal collections to empty, non-null defaults.
SDMetaUI/Models/GalleryViewModel.cs	Makes Filter nullable, adds null-guards, and initializes Rows.
SDMetaUI/Models/FilteredList.cs	Uses AllModels sentinel, makes Filter nullable, and updates query construction.
SDMeta/ImageFileLoader.cs	Updates return type to ImageFile?.
SDMeta/IImageFileLoader.cs	Changes interface to return Task<ImageFile?>.
SDMeta/CachedImageFileLoader.cs	Handles nullable loader result and avoids writing null to data source.
SDMeta/Cache/IImageFileDataSource.cs	Converts QueryParams/ModelFilter to nullable-capable records.

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[Copilot]

The PR description mentions needing to pass null! because QueryParams required non-nullable ModelFilter, but the code now defines QueryParams/ModelFilter as nullable records (ModelFilter? and string?). Update the PR description to match the current implementation so reviewers/users aren't misled about runtime null-handling requirements.

[Copilot]

OnParametersSetAsync() only updates fileSize/lastUpdated/fullPrompt/promptLines/promptFormat when ReadImageFile(...) returns non-null. If the data source returns null (missing DB entry, race during rescan, etc.), the component will keep displaying values from the previously selected file. Clear these fields when realFile == null (or before the lookup) so the UI doesn't show stale metadata.

[jamesmoore]

@copilot apply changes based on this feedback

[Copilot]

Added an else branch in OnParametersSetAsync that sets all display fields (fileSize, lastUpdated, fullPrompt, promptLines, promptFormat) to null when ReadImageFile returns null, so the panel won't show stale metadata from a previously selected file. (commit 13dae6c)

[Copilot]

Changing FileSystemChanged from FileSystemEventHandler to untyped EventHandler is a breaking public API change and also drops compile-time access to FileSystemEventArgs for subscribers (even though OnCreated/OnDeleted still raise with FileSystemEventArgs). If the intent is just nullable-sender compatibility, consider EventHandler<FileSystemEventArgs> (or a custom args type) instead of erasing the event args type.