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/sections/LayoutSection.tsx

@@ -76,7 +76,7 @@ export function LayoutSection() {
       >
         <div className={styles.shellDiagram}>
           <div className={styles.shellDiagramTop}>
-            TopBar — breadcrumb · search · env badge · shift · user avatar
+            TopBar — breadcrumb · search · filters · time range · env badge · user avatar
           </div>
           <div className={styles.shellDiagramBody}>
             <div className={styles.shellDiagramSide}>
@@ -110,7 +110,7 @@ export function LayoutSection() {
       <DemoCard
         id="topbar"
         title="TopBar"
-        description="Top navigation bar with breadcrumb, search trigger, environment badge, shift info, and user avatar."
+        description="Top navigation bar with breadcrumb, search trigger, status filters, time range, environment badge, and user avatar."
       >
         <div className={styles.topbarPreview}>
           <TopBar