fix: apply theme variables to Canvas background and drop overlay

fix: grid visibility controlled by Settings store, not toolbar event

fix: use getCurrentWebview().onDragDropEvent for native OS file drops on Windows/Tauri v2

fix: canonical Tauri OS file drag-drop handler with correct v2 payload and drop position

fix: Canvas drop handler - support library drag items + import OS files via Rust for persistent data URLs

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feat: Canvas responds to grid toggle event from toolbar

feat: Canvas supports highlighter annotations (wider, semi-transparent, screen blend) + annotation rendering improved

fix: palettes scale with board/images instead of inverse-zoom growing when zoomed out

fix: render annotations above all board images with explicit top z-index layer

refactor: Canvas.tsx - infinite canvas with pan/zoom, drag-drop, annotations, palette rendering