fix(contacts): normalize custom field type labels

[keilogic]

Summary

Fixes #244.

This updates the contact model so custom vCard field types stored as TYPE=x-... are shown to users without the transport-only x- prefix, then saved back with the prefix when they are not known vCard type values.

The Android-style vCard sample from the issue now parses TYPE=x-starý as the editable label starý, preserves standard labels such as pref and cell, and serializes an edited custom label back as TYPE=x-starý.

Validation

• npm ci
• Red check before the fix: npx ng test runbox7 --watch=false --browsers=FirefoxHeadless --include=src/app/contacts-app/contact.spec.ts failed with 2 FAILED, 23 SUCCESS because the model exposed x-starý/x-obsolete and saved TYPE=starý
• npx ng test runbox7 --watch=false --browsers=FirefoxHeadless --include=src/app/contacts-app/contact.spec.ts passed with 25 SUCCESS
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npm run lint -- --quiet
• npm run policy passed for the current commits; it still reports existing historical commit-message warnings
• git diff --check origin/master...HEAD
• git show --check --stat --oneline HEAD
• npx ng test runbox7 --watch=false --browsers=FirefoxHeadless passed with 247 SUCCESS and 2 existing skipped tests
• SKIP_CHANGELOG=1; node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; node src/build/post-build.js passed with existing Angular/CommonJS/theming warnings; build hash 391ab3d473a443c7

AI Disclosure

I used OpenAI Codex to inspect the issue and current source, make the code/test changes, and run the validation commands listed above.