test(wasm-client): cover the new save prompt, and correct a false comment

Adding `AppMode::SavePrompt` extended nothing in the browser client,
because `keys.rs::text_entry_modes` is a hand-written list and the new
mode was simply absent from it. Adding the line and watching the two
tests that consume it — typing resolves locally, backspace pops locally —
pass unchanged confirms the shared keymap already carries the mode into
the browser with no client-side change, which is the point of the keymap
move.

The comment on that helper claimed a new mode "shows up here as a missing
line rather than as untested behaviour". It does not; nothing checks the
list against `AppMode`, and `save_prompt` was silently uncovered until
added by hand. There is a second hand-written copy in
improvise-command's `text_entry_modes_with_exits`. The comment now says
what is true, and deriving both from one place is reported for filing
rather than done here — it is outside this unit's boundary.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/improvise-wasm-client/src/keys.rs

@@ -726,8 +726,16 @@ mod tests {
         KeyEvent::new(code, KeyModifiers::NONE)
     }
 
-    /// Every text-entry mode, built by its own constructor so a new one shows
-    /// up here as a missing line rather than as untested behaviour.
+    /// Every text-entry mode, built by its own constructor.
+    ///
+    /// The comment here used to claim a new mode "shows up as a missing line
+    /// rather than as untested behaviour". It does not: this is a hand-written
+    /// list and nothing checks it against `AppMode`, so a new text-entry mode
+    /// is simply absent and silently uncovered — which is what happened to
+    /// `save_prompt` (improvise-lae) until it was added by hand. There is a
+    /// second hand-written copy in `improvise-command`'s
+    /// `keymap::tests::text_entry_modes_with_exits`. Deriving both from one
+    /// place is filed, not done here.
     fn text_entry_modes() -> Vec<AppMode> {
         vec![
             AppMode::editing(),
@@ -737,6 +745,7 @@ mod tests {
             AppMode::category_add(),
             AppMode::item_add("Region".into()),
             AppMode::export_prompt(),
+            AppMode::save_prompt(),
         ]
     }