feat: implement gist creation flow

[nullcoder]

Overview

Implement the complete gist creation workflow by integrating existing UI components with the API layer to enable users to create encrypted multi-file gists.

Tasks

• Implement /create page integrating existing MultiFileEditor component
• Wire up client-side encryption using Web Crypto API
• Connect existing ExpirySelector and PasswordInput components
• Integrate form submission with POST /api/gists
• Add file validation (size limits, count limits)
• Connect ShareDialog component for success state
• Wire up existing ErrorBoundary for failure handling
• Add page-level state management

Technical Requirements

• Use existing UI components from Phase 4
• Implement client-side encryption before submission
• Handle multipart form data submission
• Validate file sizes (500KB per file, 5MB total)
• Validate file count (max 20 files)
• Show loading states during submission
• Handle all error cases gracefully

Acceptance Criteria

• User can create multi-file gists
• Files are encrypted before submission
• PIN protection works correctly
• Share URL is generated and copyable
• Errors are handled gracefully
• Form validation prevents invalid submissions
• Success state shows share dialog with URL

Dependencies

• Phase 5 APIs (complete)
• Phase 4 UI components (complete)
• Encryption utilities (complete)

References

• See docs/PHASE_6_ISSUE_TRACKING.md for detailed specifications