Feat: Add drag-and-drop reordering for primitive array fields in forms

[JoyceJ53]

Problem:
Users could add or remove items from primitive array fields (such as lists of strings or tags), but they had no way to adjust their sequence. Reordering required manually deleting elements and re-typing them from scratch, creating a frustrating user experience when index order mattered.

Solution:

• Frontend Interactivity: Integrated Sortable.js (vendored locally to maintain a zero-dependency architecture for parent apps) via a new Phoenix LiveView hook (Hooks.Sortable) to drive the drag-and-drop interface.

• User Experience & Styling: Added visual feedback rules in app.css (.sortable-ghost and .sortable-drag) to show smooth drop-zone targeting and grabbing cues.

• UI Markup Update: Modified render_fallback_attribute/7 in form.ex to inject the necessary data-sort-index metadata and introduced a dedicated grab-handle icon. This isolates the dragging action, ensuring that nested text inputs remain fully selectable and editable.

• Backend Transformation Logic: Introduced robust server-side reindexing utility functions (to_indexed_map/1 and reorder_by_indices/2) in helpers.ex to gracefully normalize gappy parameter maps and map values back onto sequential string keys.

• Form State Persistence: Implemented the core update_array_sorting event handler and sort_array_value/3 in form.ex to update and persist the newly arranged parameters directly into form.raw_params.

• Testing Pipeline: Added unit test coverage in helpers_test.exs to verify data normalization edge cases, alongside a full end-to-end integration test in page_live_test.exs to assert that simulated hook events correctly swap visual form inputs.

Contributor checklist

Leave anything that you believe does not apply unchecked.

• I accept the AI Policy, or AI was not used in the creation of this PR.
• Bug fixes include regression tests
• Chores
• Documentation changes
• Features include unit/acceptance tests
• Refactoring
• Update dependencies

Closes #395