feat(web): add MosaicLogo and MosaicSpinner components (MS15-FE-006)

- Add MosaicLogo component with 5-element Mosaic brand icon (4 corner
  squares + center circle), using CSS custom properties for theme-aware
  colors and optional rotation animation
- Add MosaicSpinner wrapper that enables spinning and supports optional
  label and full-page overlay modes
- Replace generic CSS spinner in authenticated layout loading state with
  MosaicSpinner for consistent brand identity

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/scratchpads/ms15-fe-006-mosaic-logo-spinner.md

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+# MS15-FE-006: MosaicLogo and MosaicSpinner Components
+
+## Task
+
+Create Mosaic logo icon component and spinner wrapper for use as the site-wide loading indicator.
+
+## Files to Create
+
+1. `apps/web/src/components/ui/MosaicLogo.tsx` — 5-element logo icon
+2. `apps/web/src/components/ui/MosaicSpinner.tsx` — spinner wrapper
+
+## Files to Modify
+
+1. `apps/web/src/app/(authenticated)/layout.tsx` — replace loading spinner (isLoading block only)
+
+## Design
+
+- 4 corner squares: blue (TL), purple (TR), teal (BR), amber (BL)
+- 1 center circle: pink
+- CSS vars: --ms-blue-500, --ms-purple-500, --ms-teal-500, --ms-amber-500, --ms-pink-500
+- Animation: linear 360deg rotation
+
+## Props
+
+### MosaicLogo
+
+- size?: number (default 36)
+- spinning?: boolean (default false)
+- spinDuration?: number (default 20) seconds
+- className?: string
+
+### MosaicSpinner
+
+- Wraps MosaicLogo with spinning=true
+- label?: string — optional text label below
+- fullPage?: boolean — center on screen
+
+## Status
+
+- [x] Scratchpad created
+- [ ] MosaicLogo.tsx created
+- [ ] MosaicSpinner.tsx created
+- [ ] layout.tsx updated
+- [ ] Lint clean
+- [ ] Committed and pushed