Refactor codebase into testable lib modules and thin UI layers.

[mdadul]

Summary

Behavior-preserving foundation refactor that moves shared logic into src/lib/, thins Zustand stores and React components, and adds project docs — no intentional changes to editor UX, store action semantics, shortcuts, or export output.

• Lib extractions + tests: templates (filter-templates, apply-template-to-screen), export (prepare-export-screen, export-form-state), canvas (coordinates, create-canvas-gradient), assets (persist-project-asset), elements (element-meta, selection-fields), constants barrel split, and shared test fixtures
• Component / hook splits: ExportDialog tabs + reducers, CanvasWorkspace viewport/selection/drop/text-edit/screen-context hooks, PropertiesPanel and BackgroundControls per-section components, unified TemplateCard / FloatingMenu, useProjectsCatalog for landing
• Store modularization: project-store delegates to stores/project/* (screen/element mutations, alignment, coalescing, template helpers)
• Docs: Framekit README, CONTRIBUTING.md, MIT LICENSE, GitHub repo URL fix

Line-count highlights

File	Before → After
CanvasWorkspace.tsx	~960 → ~551
PropertiesPanel.tsx	~727 → ~145
project-store.ts	~617 → ~441
ExportDialog.tsx	large monolith → tab components + hooks
BackgroundControls.tsx	~500 → ~87 (+ background/ sections)
routes/index.tsx	~142 → ~28

Test plan

• npm test — 67 tests passing
• npm run build — TypeScript + Vite production build green
• npm run lint — no new lint violations in touched files
• Manual smoke: landing page → create/open project
• Manual smoke: add/edit elements, undo/redo, template apply
• Manual smoke: export preview + PNG/ZIP download
• Manual smoke: .ssgproj import/export
• Manual smoke: screen context menu + inline text edit on canvas