fix(errors): standardize error handling patterns across modules

[Arowolokehinde]

• Replace panic!/expect() with Result returns in governance.rs and access_control.rs check_role
• Fix 5 admin functions in lib.rs silently discarding errors (let _ =) to properly return Result<(), BridgeError>
• Fix bridge.rs fee_recipient.unwrap() with proper error propagation
• Add GovernanceError enum (codes 600-605) for governance module
• Renumber CommonError codes from 400s to 500s to fix duplicate with MobilePlatformError
• Propagate check_role Result in all 15 callers across emergency.rs, bridge.rs, and audit.rs

Closes #253

🚀 Pull Request

📋 Description

🔗 Related Issue(s)

• Closes #

🎯 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)
• 📚 Documentation update
• 🔧 Tooling/Infrastructure
• 🧪 Test improvements
• 🔒 Security fix
• ♻️ Refactoring (no functional changes)
• ⚡ Performance improvements

📝 Changes Made

🧪 Testing

✅ Pre-Merge Checklist (Required)

• 🧪 Unit Tests: I have run cargo test --lib and all tests pass
• 🔨 Debug Build: I have run cargo build and the project builds successfully
• 🎯 WASM Build: I have run cargo build --target wasm32-unknown-unknown --release and WASM builds successfully
• 📝 Code Formatting: I have run cargo fmt --all -- --check and code is properly formatted
• 🔍 Clippy Lints: I have run cargo clippy and there are no new warnings

🧪 Additional Testing (Recommended)

• 📚 Documentation: I have run cargo doc --no-deps and documentation builds without errors
• 🔒 Security Audit: I have run cargo audit and no critical vulnerabilities found
• 🖱️ Manual Testing: I have tested this change manually (if applicable)
• 📊 Performance: I have verified performance impact (if applicable)

📋 Test Results

cargo test --lib
# Paste output here

cargo build --target wasm32-unknown-unknown --release  
# Paste build output here

🔍 Review Checklist

📝 Code Quality

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings or errors

🧪 Testing Requirements

• I have added/updated tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Integration tests have been updated (if applicable)

📚 Documentation

• I have updated the documentation accordingly
• I have updated the CHANGELOG (if applicable)

🔒 Security

• I have not committed any secrets, keys, or sensitive data
• I have considered security implications of my changes
• My changes do not introduce known vulnerabilities

🏗️ Contract-Specific (if applicable)

• Storage changes are backward compatible (or migration plan provided)
• Event emissions are appropriate and documented
• Error handling is comprehensive
• Gas/resource usage has been considered

📸 Screenshots/Recordings

💥 Breaking Changes

• This PR introduces breaking changes

• What breaks:
• Migration path:

📊 Performance Impact

• CPU/Memory:
• Gas costs:
• Network:

🔒 Security Considerations

• Risks:
• Mitigations:

📖 Additional Context

• Links:
• Discussions:
• Examples:

🚀 Deployment Notes

• Requires contract redeployment
• Requires data migration
• Requires configuration changes
• No deployment changes needed

📋 Reviewer Checklist

• 📝 Code review completed
• 🧪 Tests verified
• 📚 Documentation reviewed
• 🔒 Security considerations reviewed
• 🏗️ Architecture/design reviewed
• ✅ Approved for merge

---

🤖 CI Status

• 📝 Code Formatting: ✅/❌
• 🔍 Clippy Lints: ✅/❌
• 🧪 Unit Tests: ✅/❌
• 🔨 Debug Build: ✅/❌
• 🎯 WASM Release Build: ✅/❌
• 📚 Documentation: ✅/❌
• 🔒 Security Audit: ✅/⚠️

---

🎯 Ready for Review:

• Yes, all required checks pass and I'm ready for review
• No, I need to fix some issues first

---

Thank you for contributing to TeachLink! 🚀

[drips-wave]

@Arowolokehinde Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits