feat: Add Stellar address validation utility

[AbelOsaretin]

Closes #82

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional or behavioral changes)
• Performance improvement
• Documentation update
• Build / CI configuration change
• Dependency update
• Other:

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Summary

The wallet connection flow accepted any non-empty string as a secret key, deferring validation to the Stellar SDK deep in the SEP-10 flow. This caused cryptic errors ("Invalid seed") instead of clear user-facing messages. This PR adds a Validation utility class that validates Stellar secret and public key format at the UI boundary, showing immediate feedback for invalid inputs.

Motivation / Context

Fixes #82

Users who mistype their secret key receive cryptic errors from the Stellar SDK instead of clear validation messages. The issue identifies that:

• No validation utility exists (lib/utils/validation.dart returns 404)
• The wallet screen only checks for empty strings
• Invalid keys cause confusing errors during the async SEP-10 flow

Detailed Changes

• New file: lib/utils/validation.dart — Added Validation class with:

  • stellarSecretKey(String key) — validates prefix 'S', length 56, base32 character set
  • stellarPublicKey(String key) — validates prefix 'G', length 56
  • Returns null for valid keys, error message string for invalid ones

• Modified: lib/screens/wallet_screen.dart — Updated _onConnect method to:

  • Call Validation.stellarSecretKey() before passing key to provider
  • Show SnackBar with error message when validation fails
  • Prevent invalid keys from reaching WalletProvider.connect()

• New file: test/utils/validation_test.dart — Comprehensive unit tests covering:

  • Valid format keys (prefix, length, base32 characters)
  • Empty input rejection
  • Wrong prefix detection
  • Wrong length detection
  • Invalid base32 character detection
  • Whitespace trimming behavior

Current Behavior vs. New Behavior

Before: Enter any non-empty string → key passed to SDK → cryptic "Invalid seed" error deep in SEP-10 flow

After: Enter invalid key → immediate SnackBar with clear error message (e.g., "Secret key must start with 'S'", "Secret key must be exactly 56 characters (got 55)")

Testing

• Unit tests: 9 test cases covering all validation paths (valid keys, empty input, wrong prefix, wrong length, invalid base32 chars, whitespace trimming)
• All existing tests pass locally
• Manual verification: entering invalid keys now shows clear error messages instead of cryptic SDK errors

Screenshots / Recordings

N/A (CLI input validation)

Breaking Changes

No

Risks and Rollback

Low risk — validation is additive and non-breaking. Rollback: revert commits on the feature branch.

Checklist

Self-Review

• I have read the entire diff line by line as if a stranger wrote it
• No debug code remains (console.log, print, debugger, commented-out blocks)
• No hardcoded secrets, tokens, API keys, or internal URLs
• Naming is consistent with the existing codebase
• Error handling is present and produces meaningful messages
• Edge cases are addressed (null/undefined, empty collections, boundary values)
• No unused imports, dead code, or unnecessary dependencies

Testing

• All existing tests pass locally
• New tests added for new logic (functions, methods, branches)
• Edge cases and failure paths are tested, not just the happy path
• Manual testing steps documented above (if applicable)
• Screenshots / recordings attached (for UI changes)

CI / Pipeline

• All CI checks are passing (build, lint, test, type-check)
• No new compiler warnings or linting errors introduced

Documentation

• README updated if setup, usage, or installation changed
• API documentation updated for any public interface changes
• Inline comments added for non-obvious logic (explain "why", not "what")
• Configuration / env var documentation updated (if applicable)

Changelog

• Changelog entry added (if the project maintains one)
• Entry uses user-facing language, not implementation details
• Breaking changes are flagged with migration instructions

Security

• User input is validated and sanitized at trust boundaries
• No SQL injection, XSS, or injection vulnerabilities introduced
• Authentication / authorization checks are in place for new endpoints
• Dependencies have no known critical vulnerabilities

Performance

• No N+1 database query patterns introduced
• New queries use appropriate indexes
• No memory leaks (event listeners cleaned up, connections closed)
• Large data sets are paginated or streamed, not loaded entirely into memory

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Reviewer Notes

• The validation utility follows the pattern specified in the issue (#82)
• Unit tests cover all validation paths specified in the issue
• The wallet screen now shows clear error messages via SnackBar

Additional References

• Issue: https://github.com/Nodus-protocol/Nodus-Protocol-Mobile/issues/82

[AbelOsaretin]

1. CI Status ⚠️

• Analyze step fails — This is a pre-existing issue caused by --fatal-infos flag in CI workflow
• PR #104 already addresses this by changing to --fatal-warnings
• My code changes do not introduce new analysis errors

2. Merge Conflicts ✅

• No merge conflicts detected