feat: show selected word count in status bar

[willwang93]

Summary

Displays real-time word count for highlighted/selected text in the status bar.

Details

• When text is highlighted in either the rich-text editor or Markdown source mode, the counter dynamically displays X of Y words (e.g. 14 of 350 words).
• When the selection is cleared or empty, it smoothly reverts back to the standard Y words count.
• Resets selection state cleanly when switching notes.
• Tested: bunx tsc --noEmit passes cleanly and all 59 Rust tests in cargo test pass.