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.

[starc007]

Reviewed 357f47e. Found two issues to address:

1. Selected and total word counts use different rules. In rich-text mode, selecting all of # Hello world shows 2 of 3 words: the selection counts rendered text, but the total includes Markdown syntax. In source mode, selecting all of ---\ntitle: Example\n---\nHello world counts the YAML too, showing 6 of 2 words. Please use consistent counting rules for the selected text and the total, including how frontmatter is handled.

2. Switching to source mode can leave a stale selection count. Select text in a plain-text note, then switch to source mode. CodeMirror starts with an empty selection, but its callback only runs after a selection or document change, so the previous rich-text selection count can remain visible. Please report the initial selection on mount or reset the count when switching modes.

Validation: typecheck and all 37 frontend tests pass. I reproduced the count discrepancies programmatically; the mode-switch issue was traced through the code, not tested in the UI.