feat(app): display decision snapshot replay details

[JSONbored]

Parent roadmap: #127
Parent phase: #269
Parent epic: #270

Goal

Give operators and contributors a control-panel view for replaying a decision snapshot at a human-readable level.

Requirements

• Add a focused replay/details view for an existing snapshot id.
• Show source freshness, confidence context, and counterfactuals without exposing private-only fields to public contexts.
• Keep the UI scoped to inspection; do not add issue filing, PR creation, comments, labels, closure, or merge actions.

Acceptance criteria

• A user can inspect a snapshot id and understand the decision inputs.
• The view handles missing, stale, and private-only evidence cleanly.
• Public-safe and private/authenticated fields are visually and behaviorally separated.
• This is not an autonomous PR bot and not public scoring.

Validation expected

• Component or route tests for populated, stale, missing, and private-field states.
• Screenshot or recording for the changed UI state.
• npm run test:ci.