fix: stop reader key navigation leaking behind open overlays

Arrows/j/k/Home/End bubbled to the reader's window handler while a
jump/settings/action/collections/tags/context overlay was open, paging the
chapter behind it (FE-1); native select/contenteditable weren't exempt (FE-4).
New unit-tested isTypingTarget() + an overlay-open early return fix both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.13",
+  "version": "0.128.14",
   "private": true,
   "type": "module",
   "scripts": {