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/components/OptionButton.js

@@ -0,0 +1,18 @@
+import { html } from '../lib/preact.js';
+
+export function OptionButton({ number, label, description, onClick }) {
+  return html`
+    <button
+      onClick=${onClick}
+      class="group w-full rounded-xl border border-selection/70 bg-surface2/55 p-3.5 text-left transition-all duration-200 hover:-translate-y-0.5 hover:border-starting/55 hover:bg-surface2/90 hover:shadow-halo"
+    >
+      <div class="flex items-baseline gap-2.5">
+        <span class="font-mono text-starting">${number}.</span>
+        <span class="font-medium text-bright">${label}</span>
+      </div>
+      ${description && html`
+        <p class="mt-1 pl-5 text-sm text-dim">${description}</p>
+      `}
+    </button>
+  `;
+}