feat(editor): add button to insert new blocks at document end

[horacioh]

Summary

Adds a new "Add Block at End" button to the editor UI, enabling users to quickly insert new content blocks at the end of a document without needing to scroll or position their cursor manually.

Key Changes

• New Component: AddBlockAtEndButton - A floating button that intelligently manages block insertion at the document end

  • If the last block has content, creates a new empty block after it
  • If the last block is already empty, reuses it by moving cursor there
  • Cleans up leftover "/" blocks from dismissed slash menus before proceeding
  • Automatically opens the slash menu for block type selection

• Editor Integration: Integrated the button into HyperMediaEditorView component

  • Positioned below the main editor content
  • Hidden in comment/read-only mode

• Layout Adjustments:

  • Updated draft.tsx to use bottom padding (pb-60) instead of margin for better spacing
  • Applied same padding fix to resource-page-common.tsx

• Test Coverage: Added comprehensive test suite (addBlockAtEnd.test.ts) covering:

  • Block detection in single and multi-block documents
  • Insertion logic with content vs. empty blocks
  • Leftover "/" block cleanup and reuse
  • Nested block handling

Implementation Details

The button uses ProseMirror's editor state to:

1. Locate the last block in the document
2. Check if it contains content
3. Detect and remove leftover "/" blocks from previous interactions
4. Insert new blocks or reuse empty ones
5. Trigger the slash menu for block type selection