docs: add dashboard architecture to architecture.md

[OneStepAt4time]

Summary

Adds a comprehensive "Dashboard Architecture" section to covering all dashboard infrastructure as assigned by Athena (P2, milestone #14).

Section Content

1. Technology Stack — React 18 + TypeScript, Vite, React Router v6, Zustand, CSS design tokens
2. SPA Layout — Full component tree with routing hierarchy (ProtectedRoute → Layout → pages)
3. Design Token System — CSS custom properties, dark/light theme via hook, attribute
4. Component System — Table of 8 key components (Layout, ProtectedRoute, ToastContainer, ConfirmDialog, ErrorBoundary, MetricCard, SessionTable, EmptyState) with PR references
5. SSE Integration — , , hooks, live status indicator
6. API Client — Typed fetch wrapper usage example
7. Key Dashboard Pages — Table of 6 pages with routes and features

Changes

• : +97 lines (new Dashboard Architecture section)

Referenced PRs

• feat(dashboard): session history search and filter #1779 (session history search/filter)
• feat(dashboard): keyboard shortcuts for navigation and actions #1782 (keyboard shortcuts)
• feat(dashboard): sparkline mini-charts on overview MetricCards #1785 (sparklines)
• feat(dashboard): consistent empty states across all pages #1789 (empty states)
• feat(dashboard): CSV export for session history #1791 (CSV export)
• feat(dashboard): toast notification improvements #1796 (toast improvements)
• feat(dashboard): breadcrumb navigation component #1807 (breadcrumb)
• feat(dashboard): live SSE status updates on Overview page #1812 (SSE live updates)
• feat(dashboard): dark/light theme toggle #1816 (dark/light theme toggle)
• feat(dashboard): basic settings page #1825 (settings page)

Closes Athena assignment (P2, milestone #14)

Aegis version: 0.5.3-alpha
Milestone: Documentation
Assignee: Scribe

[aegis-gh-agent]

✅ Approved — dashboard architecture doc with SPA layout, design tokens, component system, SSE integration.