Add "Hide Completed" toggle to task filter bar

[Copilot]

Completed tasks clutter the task view with no way to suppress them independently of the status filter. This adds a dedicated toggle that hides completed tasks across all groupings and filter combinations.

Changes

• index.html — Added 👁 Hide Completed button to the filters row
• src/app.ts
  • hideCompleted: boolean state property on TaskManager
  • toggleHideCompleted() — flips state, updates button text/style, re-renders
  • filterTasks() — short-circuits completed tasks first when flag is set:

    if (this.hideCompleted && task.completed) return false;

• css/styles.css — .btn-secondary.active styled to primary blue to indicate the filter is on

Behaviour

The toggle is orthogonal to all other filters (category, status, group-by, search). When active, the button switches to "👁 Show Completed" with a blue highlight.

Default	Active (hiding completed)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add feature to hide completed tasks</issue_title>
<issue_description>The completed tasks can clutter up the task view. The user should have the option to hide completed tasks regardless of grouping or filtering</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add feature to hide completed tasks #34

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