Add a Ctrl+/ whole-line comment toggle to the editor

The app already treats `<!-- ... -->` as editorial notes: preprocess
strips them from the export and the word count. Ctrl+/ makes that a
one-key move, so a paragraph can be shelved without being deleted.

It grows the selection out to line boundaries first, and only toggles a
block back off when every non-blank line in it is already commented.
Blank lines are passed through so paragraph breaks survive the round
trip, and pressing it on an empty line opens `<!--  -->` with the caret
inside.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBWj9TphFMCoh7VHaSRnvQ

README.md

@@ -97,6 +97,15 @@ Terminal=false
    | `Ctrl/Cmd + E` | `inline code` |
    | `Ctrl/Cmd + Shift + X` | ~~strikethrough~~ (`~~…~~`) |
    | `Ctrl/Cmd + K` | link — `[selection](url)`, with `url` selected to replace |
+   | `Ctrl/Cmd + /` | comment the selected lines out (`<!-- … -->`), or take it back off |
+
+   `Ctrl/Cmd + /` is the odd one out: it works on whole lines, growing the
+   range out to line boundaries first, and it toggles the block off again
+   only when every non-blank line in it is already commented. Commented lines
+   stay in the file but drop out of the export and the word count, so this is
+   the way to shelve a paragraph without losing it. Blank lines in the range
+   are left as-is so paragraph breaks survive; pressing it on an empty line
+   opens `<!--  -->` with the caret inside.
 
    Press **`Ctrl/Cmd + F`** (or **Edit ▸ Find / Replace…**) to open the
    find/replace bar above the editor; **`Ctrl/Cmd + H`** opens it with the