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/mocks/metrics.ts

@@ -20,7 +20,7 @@ export interface MetricSeries {
   data: TimeSeriesPoint[]
 }
 
-// Generate a realistic time series for the past shift (06:00 - now ~09:15)
+// Generate a realistic time series for the past hours (06:00 - now ~09:15)
 function generateTimeSeries(
   baseValue: number,
   variance: number,
@@ -44,12 +44,12 @@ function generateTimeSeries(
 // KPI stat cards data
 export const kpiMetrics: KpiMetric[] = [
   {
-    label: 'Exchanges (shift)',
+    label: 'Exchanges',
     value: '3,241',
     trend: 'up',
     trendValue: '+12%',
     trendSentiment: 'good',
-    detail: '97.1% success since 06:00',
+    detail: '97.1% success rate',
     accent: 'amber',
     sparkline: [28, 32, 29, 35, 38, 41, 37, 44, 42, 47, 45, 51, 48, 52],
   },
@@ -64,12 +64,12 @@ export const kpiMetrics: KpiMetric[] = [
     sparkline: [98.2, 97.9, 98.1, 97.8, 97.5, 97.6, 97.4, 97.2, 97.3, 97.1, 97.0, 97.1, 97.2, 97.1],
   },
   {
-    label: 'Errors (shift)',
+    label: 'Errors',
     value: 38,
     trend: 'up',
     trendValue: '+5',
     trendSentiment: 'bad',
-    detail: '23 overnight · 15 since 06:00',
+    detail: '38 errors in selected period',
     accent: 'error',
     sparkline: [1, 2, 1, 3, 2, 4, 3, 5, 4, 6, 5, 7, 6, 8],
   },