docs(transcription): explain why @mention mirror uses \$state+\$effect

The mirror effect on the dropdown's searchQuery looks like it should be
\$derived but it cannot be: bind:value on the <input> writes to the same
state, so it must remain mutable. Felix #2 on PR #629.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.svelte

@@ -34,6 +34,12 @@ let searchQuery = $state(untrack(() => editorQuery));
 let userHasEdited = $state(false);
 
 // Mirror the editor's typed text until the user takes ownership.
+//
+// Why `$state + $effect` (not `$derived`): `searchQuery` is also written by
+// `bind:value` on the <input> below, so it needs to be a mutable `$state`.
+// A `$derived` would be read-only and would clobber direct user edits on
+// every editor keystroke. The `userHasEdited` latch pins ownership once the
+// user types into the input. Felix #1 on PR #629.
 $effect(() => {
 	if (!userHasEdited) {
 		searchQuery = editorQuery;