refactor(dashboard): extract modular Preact component structure

Replace the monolithic single-file dashboards (dashboard.html,
dashboard-preact.html) with a proper modular directory structure:

  dashboard/
    index.html              - Entry point, loads main.js
    main.js                 - App bootstrap, mounts <App> to #root
    styles.css              - Global styles (dark theme, typography)
    components/
      App.js                - Root component, state management, polling
      Header.js             - Top bar with refresh/timing info
      Sidebar.js            - Project tree navigation
      SessionCard.js        - Individual session card with status/actions
      SessionGroup.js       - Group sessions by project path
      Modal.js              - Full conversation viewer overlay
      ChatMessages.js       - Message list with role styling
      MessageBubble.js      - Individual message with markdown
      QuestionBlock.js      - User question input with quick options
      EmptyState.js         - "No sessions" placeholder
      OptionButton.js       - Quick response button component
      SimpleInput.js        - Text input with send button
    lib/
      preact.js             - Preact + htm ESM bundle (CDN shim)
      markdown.js           - Lightweight markdown-to-HTML renderer
    utils/
      api.js                - fetch wrappers for /api/* endpoints
      formatting.js         - Time formatting, truncation helpers
      status.js             - Session status logic, action availability

This structure enables:
- Browser-native ES modules (no build step required)
- Component reuse and isolation
- Easier styling and theming
- IDE support for component navigation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

dashboard/main.js

@@ -0,0 +1,7 @@
+// Dashboard entry point
+import { render } from './lib/preact.js';
+import { html } from './lib/preact.js';
+import { App } from './components/App.js';
+
+// Mount the app
+render(html`<${App} />`, document.getElementById('app'));