Add validation for credential type selection in RequestCredential form

[Copilot]

Addresses feedback from PR #225: users could submit the credential issuance form without selecting a credential type, sending an empty string to the API.

Changes:

• Added validation in issueCredential() to check selectedCredentialType is non-empty before API call
• Error displays "Please select a credential type" when validation fails
• Validation executes before issuer lookup, following same pattern as existing selectedWalletDID validation

issueCredential() {
  this.issueError = undefined
  
  if (!this.selectedCredentialType) {
    this.issueError = 'Please select a credential type'
    return
  }

  const issuerDID = this.getIssuerForType(this.selectedCredentialType)
  // ... rest of validation and API call
}

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