Create task screen — URL field #124

Description

Add an optional URL field to the Create Task screen so users can attach a relevant link to a task.

This extends the Create Task screen from #121 and persists the value into Task.url from #120. The URL is optional. Blank input should persist as null. Non-blank input should be validated before saving so invalid links are not stored.

Scope

In scope: optional URL input on the Create Task screen, basic URL validation, saving valid URLs into Task.url, persisting blank input as null, validation error display, string resources, and ViewModel tests for blank, valid, and invalid URL states.

Out of scope: making URLs tappable, URL preview cards, metadata fetching, link icons in task rows, and task detail rendering.

Acceptance criteria

• GIVEN the user opens the Create Task screen
  WHEN the form is displayed
  THEN the URL field is empty by default
• GIVEN the user leaves the URL field blank
  WHEN they save the task
  THEN Task.url is persisted as null
• GIVEN the user enters a valid URL
  WHEN they save the task
  THEN the URL is persisted into Task.url
• GIVEN the user enters an invalid URL
  WHEN they tap save
  THEN a validation error is shown and the task is not saved
• GIVEN URL UI text or validation messages are displayed
  WHEN the app renders the Create Task screen
  THEN all user-visible text comes from string resources