fix: Stop mobile editor from reverting keystrokes

currentText was $state, and the validated-state sync $effect both reads
and writes it. Every keystroke (which sets currentText in the CodeMirror
updateListener) therefore re-ran the effect while re-validation was
still in flight, dispatching a full-document replacement with the stale
validatedState text — visibly reverting the keystroke and resetting the
cursor until validation caught up.

The effect only needs to react to validatedState publishes, so make
currentText a plain variable, matching DesktopEditor.

No unit test: the repo has no component-test harness, and CodeMirror
cannot mount under jsdom without one.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/components/MobileEditor.svelte

@@ -15,7 +15,10 @@
 
   let editorView: EditorView | undefined;
   let editorContainer: HTMLDivElement;
-  let currentText = $state('');
+  // Deliberately not $state: the sync effect below both reads and writes it,
+  // so a reactive currentText would make every keystroke re-run the effect
+  // against the not-yet-revalidated state and revert the user's input.
+  let currentText = '';
   const themeCompartment = new Compartment();
   const languageCompartment = new Compartment();