Add task count badges to sidebar navigation items

[MeisnerDan]

Display count badges next to navigation items showing how many tasks are in each view. For example, "In Progress (5)" or "Do Quadrant (12)" helps users quickly assess workload at a glance. This is a visual enhancement that improves information scannability.

Acceptance Criteria

• Count badges appear next to sidebar navigation items (Projects, Goals, Activity, etc.)
• Badges update in real-time as tasks are created, deleted, or moved
• Badge styling follows shadcn/ui Badge component conventions
• Badges respect task filters and kanban status (e.g., only count tasks in current view)
• Works with responsive design (badges scale appropriately on mobile)
• Unit tests verify badge count accuracy across state changes