Rewrite in-app docs for current UI and features

Replace all hardcoded documentation content to match the current app:
- Updated terminology throughout (Secret key, AI Brain, Server address)
- New sections for learning strategies, audio, paste notes, search toggle
- Preparation phase and auto-summarise documented
- Troubleshooting updated for new flow and common issues
- Technical overview reflects current architecture including strategies,
  speech utilities, and summarisation endpoint
- Removed stale Icon Generation and API Reference sections
- Simplified navigation to 8 sections (was 10)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Michael Borck