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.h

@@ -94,6 +94,27 @@ void frame_config_set_last_display_crc32(uint32_t crc32);
  */
 void frame_config_invalidate_last_display_crc32(void);
 
+/**
+ * Returns the hold_duration_ms the server most recently reported via GET
+ * /frame/config (see frame_client.c's fetch_frame_config/frame_client_run)
+ * -- how long NEXT/BACK must be held before next_button_check()/
+ * back_button_check() treat it as a hold-for-global-action instead of a
+ * short press. Returns ESP_ERR_NVS_NOT_FOUND if the device has never
+ * fetched one yet (fresh install/factory reset); caller should fall back
+ * to CONFIG_FRAME_HOLD_ACTION_MS in that case.
+ *
+ * Deliberately a *previous* cycle's value: this cycle's own button
+ * decision happens in main.c before WiFi even connects, but
+ * /frame/config isn't fetched until near the end of frame_client_run
+ * (after the image fetch, for connection-warmth/timeout reasons -- see
+ * its own comment) -- so there's no same-cycle fresh value to use yet.
+ */
+esp_err_t frame_config_get_hold_duration_ms(uint32_t *out);
+
+/** Persists the hold duration reported by the server, for the *next*
+ * boot's button-hold decision to use. */
+void frame_config_set_hold_duration_ms(uint32_t ms);
+
 /**
  * Returns this device's provisioning AP identity: a fixed SSID (from
  * Kconfig) and a password that's generated once on first use and persisted