v0.4.12: Code Quality — Async Panels, Final Classes, Stored Paths

v0.4.12 — Code Quality Improvements (Audit Cycle 4)

High Priority

• StoryStudioViewModel: Replaced blocking NSOpenPanel.runModal() with async panel.begin{} in both importReferenceImage() overloads — prevents main actor blocking when importing reference images for characters and settings

Low Priority

• StoryflowInstructionGenerator, StoryflowValidator: Marked final for better Swift dispatch performance
• ConfigPresetsManager: Promoted presetsDirectory and presetsFilePath from computed properties to stored let constants — avoids repeated FileManager.urls() lookups on every access
• PromptStyleManager: Promoted stylesDirectory and stylesFilePath to nonisolated let stored constants — same optimization, also removes unnecessary FileManager.default allocation per call