Implement InputValidator helper class

[zaelgohary]

Changes:

• Add InputValidator class with URL validation, JSON decoding, and
  content fetching with size limits
• Refactor sign screens (QR code, link, sign screen) to use InputValidator
• Improve URL validation with automatic whitespace trimming
• Add size limits for URLs (2048 chars), content (100KB), and scope (10KB)
• Enhance error handling with consistent validation patterns
• Refactor signing_mixin to reuse InputValidator for dest URL validation
• Improve stellar secret validation with whitespace trimming
• Remove unused imports (dart:convert from uni_link_service)

#1141