[6/5] Post‑E2E refactoring: hooks, memoization, context split, and accessibility tidy‑ups

[mfittko]

Parent: #7

Scope
Apply refactors after E2E coverage is green to simplify components, reduce re‑renders, and align with coding best practices.

Deliverables

• GeometryCanvas
  • Extract hooks: useGridSync (external↔internal sync + drag guards), useFormulaSelection (point selection/navigation), useMeasurementsPanel (read/update)
  • Split render into ShapeLayers and FormulaLayer
  • Memoize heavy computations; wrap FormulaGraph/ShapeRenderer in React.memo
  • Replace magic numbers/timeouts with named constants (e.g., GRID_POSITION_DEBOUNCE_MS)
  • Centralize calibration/localStorage via helper
• CanvasGrid
  • Debounce origin updates via small util; extract z‑index constants
• ShareViewOptions context
  • Split isSharePanelOpen into separate context; memoize provider values; updateShareViewOption via useCallback
• URL helpers
  • Consolidate boolean parsing utilities; ensure deterministic param ordering; unit tests for env‑overridden defaults (admin)
• UnifiedSettingsModal / SharePanel
  • ARIA labels for icon buttons; lazy render tab content; extract ToggleRow/OptionRow
• GlobalControls / Toolbar
  • Ensure accessible names; unify IconButton pattern; memoize handlers
• Logging/ESLint
  • Gate verbose logs behind loggingEnabled/NODE_ENV; narrow react‑refresh overrides

Acceptance Criteria

• No functional regressions; E2E remains green
• Drag remains smooth (no remounts on drag); no snap‑back
• Component tree size reduced; fewer renders on drag and toggle changes
• Lint clean; unit tests added for new helpers