feat: implement one-time view gists

[nullcoder]

Description

Create single-view gists that auto-delete after viewing.

Priority

HIGH - Critical security feature for sensitive code sharing.

Tasks

• Add one-time option to create form
• Implement warning modal before viewing
• Create secure deletion after decryption
• Add download before view option
• Handle concurrent access attempts
• Implement view tracking
• Add visual indicators
• Create audit logging

Technical Requirements

• One-time flag must be stored in gist metadata
• Implement atomic read-and-delete operation
• Handle race conditions for concurrent access
• Ensure complete deletion from R2
• Add clear warnings before viewing
• Support "download without viewing" option
• Implement view attempt logging

Acceptance Criteria

• One-time gists delete after viewing
• Warning is clear and prominent
• Download option works before deletion
• Concurrent access is handled
• Deletion is immediate and complete
• No data remnants remain in R2
• User understands consequences before viewing
• Audit trail exists for security

Dependencies

• Issue feat: implement R2 storage foundation #111 (View Flow)
• Existing components:
  • Dialog (for warning modal)
  • Button (for actions)
  • Badge (for one-time indicator)

Security Considerations

• Implement server-side view tracking
• Use database transactions or R2 conditional requests
• Ensure idempotent deletion
• Add rate limiting to prevent abuse
• Consider adding view token for extra security
• Log all access attempts (success and failure)

Implementation Details

// API endpoint modification
async function getGist(id: string) {
  const gist = await fetchGist(id);
  
  if (gist.oneTime) {
    // Atomic operation
    await Promise.all([
      deleteGist(id),
      deleteBlobs(gist.files),
      logViewEvent(id)
    ]);
  }
  
  return gist;
}

UI Flow

1. User navigates to one-time gist
2. Warning modal appears
3. Options: "View and Delete" or "Download Only"
4. If view: decrypt, display, then delete
5. If download: provide files without displaying
6. Show deletion confirmation

Estimated Time

3-4 days

References

• Phase 6 Tracking: docs/PHASE_6_ISSUE_TRACKING.md (Issue feat: create comprehensive content pages and update navigation #116)
• Security Guidelines: docs/SECURITY.md
• API error handling patterns