UX: Add empty state with task generation CTA to task board

[frankbria]

Problem

TaskBoardView with zero tasks shows a loading state but transitions to an empty board with no explanation or guidance. There is no empty state message or next-action prompt.

Impact

New workspaces with a PRD but no generated tasks leave users stuck. The task board appears broken rather than empty-by-design.

Recommendation

1. Add empty state card when tasks.length === 0 after loading completes
2. Text: "No tasks yet. Generate tasks from your PRD or create one manually."
3. Primary CTA: 'Generate from PRD →' (links to /prd)
4. Secondary CTA: 'Create Task' (opens new task dialog)
5. Show PRD status context: "PRD found" vs. "No PRD — start there first"

Acceptance Criteria

• Empty state renders when task list is empty (not loading)
• Empty state explains how to create tasks
• Links to /prd if workspace has a PRD
• Shows PRD availability in empty state context
• Does not flash during initial load

Affected Components

web-ui/src/components/tasks/TaskBoardView.tsx

[coderabbitai]

🔗 Related PRs

#194 - Fix PRD generation progress tracking UI [merged]
#213 - feat(planning): Add Generate Task Breakdown button to DiscoveryProgress [merged]
#215 - feat(dashboard): Add phase-aware task view integration (#016-6) [merged]
#389 - [Phase 3] Task Board: bulk stop, reset, and state management actions [merged]

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[traycerai]

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