fix: timelapse download robustness + device-switch + open-folder

[SukiSunYuhang]

Description

fix:

• Invalid download path (e.g. USB removed): bail out before downloading instead of crashing on file write
• Cancel still completes: WAN cancel now erases g_wan_pending and the deferred sw_NotifyUploadTimelaspe kickoff checks skipped_indices, so a cancelled file can't be restarted; cancel callback runs cancel_download immediately (no CallAfter window)
• Delete a downloading file: sw_DeleteCameraTimelapse aborts the matching PC download by date_index
• Progress regression on weak network: on_progress percent is monotonic (no backward jump from dlnow fluctuation)
• Device switch (sw_mqtt_set_engine): cancel_all_active_timelapse stops every in-flight download and marks each unfinished file as "Download Failed"; on_error skipped branch skips mark when ctx->cancelled so the failed state isn't overridden by "cancelled"
• New sw_OpenTimelapseFolder interface + desktop_open_folder helper: open the containing folder even when the file was deleted externally (sw_FileView kept unchanged for other callers)