fix: use pixel dimensions for wallpaper images

[Grumpicus]

Fixes image sizing for files whose DPI metadata causes NSImage.size to report point dimensions instead of actual pixels.

Example: a 7680x1440 JPEG with 1 DPI metadata was shown as 552960x103680+ in the editor.

Changes:

• Add an NSImage.pixelSize helper using NSBitmapImageRep.pixelsWide/pixelsHigh, falling back to CGImage.width/height
• Use pixel dimensions for editor labels, fit-to-canvas math, canvas preview sizing, thumbnails, and render sizing

Validation:

xcodebuild -scheme SpreadPaper -configuration Debug -derivedDataPath /tmp/SpreadPaperForkDerivedData build

[Grumpicus]

Small note on the nonisolated changes:

This project has SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor, so file-level functions/properties are inferred as main-actor isolated unless marked otherwise.

pixelSize is used from WallpaperManager.renderForScreen, which is already nonisolated, so the property also needs to be explicitly nonisolated.

renderThumbnails(jobs:) is called from a Task.detached thumbnail-rendering path, so it also needs to be callable outside the main actor. The previous @Sendable annotation conflicts with the inferred main-actor isolation and triggers a Swift 6 diagnostic. Making the function nonisolated is the relevant fix here; the detached task closure remains the concurrency boundary, and ThumbnailJob / ThumbnailResult already cover the sendability requirements for the values crossing that boundary.