feat(DM): library UI with episode list, SD status, cache, close button, stable single-file DL

- DiscoveryMountain: new UI optimized for episode list (not player anymore)
  - Shows episodes for current season, indicates if on SD card (green=On SD, gray=Not DL)
  - Per-episode action: download single via SdDownloader or play via Mp3Player
  - DL Missing button for missing eps in season (currently single first missing, batch reverted)
  - Season selector screen with cached/total counts
  - Top-right X close button
  - Seasons/episodes cached to cache.json to avoid network fetch every launch
  - Fixes: overlay deletion race (deferred timer), ui_timer leak causing crash on close, season selection overwritten by fetch task
  - Fixes: unicode ellipsis squares

- SdDownloader: revert to stable single-file mode (batch array caused crashes)
  - Remove batch fields and array allocation, keep single url/path download
  - Result bundle simple success/path/bytes

- Mp3Player: add resume position support (pos_sec from bundle, total_sec estimate, total_decoded_samples), return position on exit via bundle for DM to save

Tested on 192.168.68.133: opens, shows S10 2/6, Play/DL buttons, close works, no crash on open/close cycles, cache shows 'Loaded from cache, refreshing...'

Apps/DiscoveryMountain/CMakeLists.txt

@@ -0,0 +1,12 @@
+cmake_minimum_required(VERSION 3.20)
+include($ENV{IDF_PATH}/tools/cmake/project.cmake)
+if (DEFINED ENV{TACTILITY_SDK_PATH})
+    set(TACTILITY_SDK_PATH $ENV{TACTILITY_SDK_PATH})
+else()
+    set(TACTILITY_SDK_PATH "../../release/TactilitySDK")
+    message(WARNING "TACTILITY_SDK_PATH not set, defaulting to ${TACTILITY_SDK_PATH}")
+endif()
+include("${TACTILITY_SDK_PATH}/TactilitySDK.cmake")
+set(EXTRA_COMPONENT_DIRS ${TACTILITY_SDK_PATH})
+project(DiscoveryMountain)
+tactility_project(DiscoveryMountain)