feat: implement contextual help system with searchable overlay
- Added '?' button in expert mode header that opens help overlay
- Created dashboard/help_articles.json with 74 help articles covering:
* Basics (sensing links, Fresnel zones, CSI technology, passive radar, etc.)
* Setup (GDOP coverage, node placement, floor plan calibration, etc.)
* Features (automations, fall detection, BLE identity, predictions, etc.)
* Interface (command palette, simple mode, timeline, notifications, etc.)
* Troubleshooting (detection quality, false positives, node offline, etc.)
* Advanced (diurnal baseline, time-travel, explainability, etc.)
* Integration (MQTT, Home Assistant, etc.)
* Maintenance (re-baseline, OTA update, backup/restore, etc.)
- Help overlay includes fuzzy search (same as command palette)
- Category filter buttons for easy navigation
- Each article has title, 1-3 sentence explanation, and optional action link
- No server round-trip - all data loaded from static JSON
- Keyboard shortcut: Ctrl+? or Cmd+?
- Styled consistently with existing dashboard UI
Acceptance criteria met:
- Help overlay opens with '?' button
- Search finds relevant articles (test 'fresnel' -> Fresnel article appears)
- 74 articles covering major features (exceeds 30-50 requirement)
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