fix(DF-889): enhance postcode validation and add error handling in ma…

[mokhld]

…nual address

Proposed change

• Add postcode format validation to manual address entry in the postcode lookup journey.
• Invalid postcodes now show “Enter a valid postcode” error instead of being accepted.

Jira ticket: https://eaflood.atlassian.net/browse/DF-889?atlOrigin=eyJpIjoiM2U0OGE5ODgxYzI2NDllN2I5ODRmYTBmOWFlYWM2NWYiLCJwIjoiaiJ9

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud