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.

firmware/main/wifi_provisioning.c

@@ -234,6 +234,29 @@ void frame_config_invalidate_last_display_crc32(void)
     nvs_close(handle);
 }
 
+esp_err_t frame_config_get_hold_duration_ms(uint32_t *out)
+{
+    nvs_handle_t handle;
+    esp_err_t err = nvs_open(NVS_NAMESPACE, NVS_READONLY, &handle);
+    if (err != ESP_OK) {
+        return err;
+    }
+    err = nvs_get_u32(handle, "hold_ms", out);
+    nvs_close(handle);
+    return err;
+}
+
+void frame_config_set_hold_duration_ms(uint32_t ms)
+{
+    nvs_handle_t handle;
+    if (nvs_open(NVS_NAMESPACE, NVS_READWRITE, &handle) != ESP_OK) {
+        return;
+    }
+    nvs_set_u32(handle, "hold_ms", ms);
+    nvs_commit(handle);
+    nvs_close(handle);
+}
+
 /* ------------------------------------------------------------------------
  * WiFi fast-connect cache
  * ---------------------------------------------------------------------- */