feat: add ButtonGroup, theme toggle, dark theme fixes, remove shift references

- Add ButtonGroup primitive: multi-select toggle with colored dot indicators
- Replace FilterPill status filters with ButtonGroup in TopBar and EventFeed
- Add light/dark mode toggle to TopBar (moon/sun icon)
- Fix dark theme: add --purple/--purple-bg tokens, replace all hardcoded
  #F3EEFA/#7C3AED with tokens, fix --amber-light text contrast in sidebar,
  brighten --sidebar-text/--sidebar-muted tokens, use color-mix for
  ProcessorTimeline bar fills
- Remove all "shift" references (presets, labels, badges)
- Shrink SegmentedTabs height to match search bar and ButtonGroup
- Update COMPONENT_GUIDE.md with new components and updated descriptions
- Add ButtonGroup demo to Inventory
- Add README.md with setup instructions and navigation guide

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/pages/Inventory/Inventory.tsx

@@ -13,6 +13,7 @@ const NAV_SECTIONS = [
       { label: 'Avatar', href: '#avatar' },
       { label: 'Badge', href: '#badge' },
       { label: 'Button', href: '#button' },
+      { label: 'ButtonGroup', href: '#buttongroup' },
       { label: 'Card', href: '#card' },
       { label: 'Checkbox', href: '#checkbox' },
       { label: 'CodeBlock', href: '#codeblock' },