feat: extract shared KeyboardNav utility from duplicated %raw blocks

Five screens had identical %raw keyboard event handler patterns
(addEventListener/removeEventListener/key extraction). Extract
into src/app/utils/KeyboardNav.res for reuse.

Part of #28.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hyperpolymath

src/app/utils/KeyboardNav.res

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: PMPL-1.0-or-later
+// KeyboardNav — shared keyboard event handler utilities for IDApTIK screens.
+// Eliminates duplicated %raw addEventListener/removeEventListener blocks.
+
+/// Register a keydown handler on window. Returns an opaque handler token
+/// that must be passed to removeKeydownHandler for cleanup.
+let addKeydownHandler: (({..}) => unit) => {..} = %raw(`
+  function(fn) {
+    var h = function(e) { fn(e); };
+    window.addEventListener('keydown', h);
+    return h;
+  }
+`)
+
+/// Remove a previously registered keydown handler (pass the token from addKeydownHandler).
+let removeKeydownHandler: ({..}) => unit = %raw(`
+  function(handler) {
+    window.removeEventListener('keydown', handler);
+  }
+`)
+
+/// Extract the key name from a keyboard event object.
+let eventKey: ({..}) => string = %raw(`function(e) { return e.key; }`)
+
+/// Call preventDefault() on a keyboard event object.
+let preventDefault: ({..}) => unit = %raw(`function(e) { e.preventDefault(); }`)