Show reasoning tooltip for each suggested deadline in DeadlineSuggester

[Kingsman-99]

Description

DeadlineSuggester lists suggested deadline dates but gives no explanation for why each date was chosen. A tooltip on each suggestion (e.g. "Based on your average payment cycle of 14 days") helps users understand and trust the suggestions.

Acceptance Criteria

• Each deadline suggestion has a hover/focus tooltip explaining the reasoning
• Tooltip content is derived from the suggestion metadata (reason string)
• Tooltips are accessible via keyboard focus as well as mouse hover
• Existing tests continue to pass

Context

• Target file: src/components/DeadlineSuggester.tsx