Surface Collection Item appearances in Filament: typed pivot model, shared helpers, and new relation managers (#1286)

Surface Collection Item appearances in Filament: typed pivot model, shared helpers, and new relation managers (#1286)

* Initial plan

* Add typed CollectionItem pivot model and Filament collection appearances support

- Add App\Models\CollectionItem typed pivot model with contextual description and source provenance helpers
- Update Collection::attachedItems() and Item::attachedToCollections() to use ->using(CollectionItem::class)
- Add App\Filament\Support\CollectionItemAppearance with shared display order column, contextual text preview column, language keys column, and view appearance text action
- Update CollectionResource ItemsRelationManager with display order sorting, contextual text columns, and view appearance action
- Replace DisplayedInCollectionsRelationManager with CollectionAppearancesRelationManager on ItemResource
- Add 16 unit tests for CollectionItem model and 8 Filament tests for appearance visibility

Agent-Logs-Url: https://github.com/metanull/inventory-app/sessions/9a59f5c8-9d0f-431d-a9b5-17ed498b2e49

Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>

* Address code review feedback: null-safety, variable naming, and test comments

Agent-Logs-Url: https://github.com/metanull/inventory-app/sessions/9a59f5c8-9d0f-431d-a9b5-17ed498b2e49

Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>

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Fixes #1280
Fixes #1281
Fixes #1282
Fixes #1283
Fixes #1284