feat: focus editor and move cursor to end on padding area click

[j4rviscmd]

Summary

• Clicking the bottom padding zone (pb-[60vh]) of the editor previously blurred the editor because the click target was outside the .bn-editor contenteditable region
• Now clicking anywhere in the wrapper padding focuses the editor and places the cursor at the end of the last block, matching VSCode-like behavior

Test plan

• Click the bottom padding area below editor content → editor focuses, cursor moves to end of last block
• Click inside editor content → normal selection/editing behavior unchanged
• Formatting toolbar, link toolbar, and search panel still work normally
• Verify on both light and dark themes

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