fix(measure): supervisor modal — backspace works and fields reset

Two issues with the "Autorizzazione capoturno" modal:

1. Backspace and digits did not work in the username/password fields. The
   numpad component installs a window-level keydown handler (for the USB
   caliper / keyboard wedge) that preventDefault()s digits, Backspace, Enter
   etc. It swallowed those keys inside the modal inputs. Guard handleKeydown
   to ignore events whose target is an editable field (INPUT/TEXTAREA/SELECT/
   contentEditable); the numpad has no text inputs of its own.

2. The modal kept the previous username/password: opening only set the flag,
   and closing via backdrop click did not clear the fields. Add
   openSupervisorModal()/closeSupervisorModal() that always reset username,
   password and error, and use them for every open (fermo_linea,
   fine_produzione, out_of_tolerance) and close (backdrop, Annulla).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/frontend/flask_app/static/js/numpad.js

@@ -163,6 +163,17 @@ function numpad() {
      * @param {KeyboardEvent} e - The keyboard event
      */
     handleKeydown(e) {
+      // Ignore keystrokes aimed at an editable field (e.g. the supervisor
+      // login modal). The numpad has no text inputs of its own, so this
+      // window-level capture is only meant for the USB caliper / keyboard
+      // wedge when no field is focused. Without this guard the numpad would
+      // swallow digits and Backspace inside those inputs.
+      const t = e.target;
+      if (t && (t.tagName === 'INPUT' || t.tagName === 'TEXTAREA' ||
+                t.tagName === 'SELECT' || t.isContentEditable)) {
+        return;
+      }
+
       // Number keys
       if (e.key >= '0' && e.key <= '9') {
         e.preventDefault();