docs: add comprehensive v4.0 feature planning document

Add detailed feature proposal for v4.0 major release based on:
- Comprehensive codebase review of current v3.1.0 implementation
- Market research of 2025 task management app trends
- Analysis of feature gaps vs leading productivity apps

Document includes:
- 15 features across 3 priority tiers (Critical, High-Value, Nice-to-Have)
- Detailed technical specifications with database schema changes
- Implementation roadmap with 4 phases (16-24 weeks total)
- User stories and example use cases for each feature
- Success metrics and KPIs for measuring adoption
- Risk assessment and mitigation strategies

Key proposed features:
- Tier 1 (Critical): Focus Sessions/Pomodoro, Time Blocking/Calendar View,
  Task Templates, Natural Language Quick Entry
- Tier 2 (High-Value): Calendar Integration (iCal), Goal Tracking/OKRs,
  Workspace Projects, Enhanced Notifications
- Tier 3 (Nice-to-Have): Voice Input, Habit Tracker, Client-side AI Assistant

All features maintain privacy-first, offline-first architecture.
Includes database migration path from v6 to v7.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: vscarpenter