android: ticking a box on a card threw the editor open on top of it

Reported from the device: tapping a checkbox on the board checks it AND opens the
note. The "and" is the tell — both things happened, so this was never a tap landing
on the wrong target.

`mutate` ends with `editing = updated ?: state.editing`. That is right for an editor
action, where the reloaded note refreshes a screen already on display. But
`editing != null` IS "the editor is up" — it is what MainActivity's `when` selects
on — so calling `mutate` from the BOARD, where editing is null, wrote the mutated
note into it and opened the editor as a side effect of saving.

Fixed at `mutate` rather than at the caller, because the caller was not wrong: any
board-initiated mutation would have done this, and toggleItem is simply the first
one to exist. It now refreshes an open editor and cannot open a closed one.

The comment claimed the narrower behaviour all along — "so an open editor shows its
own change" — which is what the code should have been doing and wasn't.

android/app/src/main/java/com/fabledsword/thoughtsync/ui/BoardViewModel.kt

@@ -413,9 +413,10 @@ class BoardViewModel(
     /**
      * The one path every store mutation takes.
      *
-     * Each core mutation returns the reloaded note, which goes straight into
-     * [BoardState.editing] so an open editor shows its own change without a
-     * re-query. The BOARD list is then reloaded rather than patched in place:
+     * Each core mutation returns the reloaded note, which refreshes
+     * [BoardState.editing] so an OPEN editor shows its own change without a
+     * re-query — and does nothing at all when the editor is closed, because that
+     * field doubles as "which screen is up". The BOARD list is then reloaded rather than patched in place:
      * pinning re-sorts it, archiving removes the note from it, and adding a label
      * can move it in or out of a label view — a splice would have to reimplement
      * the core's ordering and membership rules in Kotlin to get any of that right.
@@ -451,7 +452,12 @@ class BoardViewModel(
                     withContext(Dispatchers.IO) { onRemindersChanged() }
                     state.copy(
                         notes = notes,
-                        editing = if (closeEditor) null else updated ?: state.editing,
+                        // Only REFRESHES an open editor; it must never open one.
+                        // `editing != null` IS "the editor is on screen", so writing
+                        // the reloaded note in unconditionally meant any mutation
+                        // started from the BOARD threw the editor open on top of it —
+                        // which is exactly what ticking a checkbox on a card did.
+                        editing = if (closeEditor) null else state.editing?.let { updated ?: it },
                         saving = false,
                         error = null,
                     )