* Perf: cache label collection, offload .fig compression to worker
Label cache: collect sections/components once per scene change,
filter by viewport on each frame. Eliminates full tree walk during
pan/zoom (~17ms/frame → <1ms on large files).
Export worker: move fflate compression off the main thread to prevent
frame drops during save (451ms+ → non-blocking).
* Perf: worker-based .fig parsing, instance index, non-blocking font loading
- Offload .fig parsing (unzip + Kiwi decode) to a Web Worker
- Add instance index (componentId → Set<nodeId>) for O(1) getInstances()
- Defer graph event subscription during file open to skip redundant syncs
- Make font loading non-blocking — render immediately, load fonts in background
- Copy image buffers before worker transfer to prevent detached ArrayBuffer crash
- Show toast on font load failure and file open errors
- Cache failed Google Fonts families to avoid repeated network requests
- Fix missing ref import in FillPicker
- Yield to UI between parse and layout for responsive loading spinner
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