Move button actions to per-widget config, add hold-for-global-action

Next/back button assignment moves from a frame-level "Button
assignments" card into each widget's own gear-icon dialog, prefilled
with a sane default at creation (photos/calendar -> advance/back,
whiteboard/weather -> check_now, others -> none). At most one binding
per (widget, button) now -- cross-widget execution order never
mattered since each widget's action only touches its own state.

New firmware capability: holding NEXT or BACK past a configurable
duration (min 3s, server-side default) triggers a frame-wide action
instead of the per-widget short-press one -- cycling saved layouts,
refreshing all widgets, or freezing/unfreezing every photo widget (see
app/global_actions.py). Firmware next/back checks gain the same
hold-duration polling the combo button already had; the threshold
comes from the previous wake's /frame/config fetch (persisted in NVS),
since this wake's button decision happens before that request.

Not done here: firmware/version.txt is intentionally left unbumped --
this hasn't been built or hardware-tested (no ESP-IDF toolchain in this
environment), so no firmware release build should be triggered yet.

server/app/static/widget_dialog_tasks.js

@@ -89,6 +89,7 @@ function initTasksDialog() {
   document.getElementById('tasks-preview-refresh').addEventListener('click', loadTasksPreview);
   loadTasksPreview();
   initBorderFields();
+  initButtonActionFields();
 }
 
 function closeTasksDialog() {