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/main/Source/download.c

@@ -0,0 +1,49 @@
+#include "download.h"
+#include "storage.h"
+#include <tt_lvgl.h>
+#include <string.h>
+#include "esp_log.h"
+
+// Simplified: internal download removed, we use external SdDownloader app
+// Keep overlay functions as stubs for compatibility, but they do minimal
+
+void hide_dl_overlay(){
+    if(G.dl_overlay){
+        lv_obj_delete(G.dl_overlay);
+        G.dl_overlay=NULL;
+        G.dl_bar=NULL;
+        G.dl_lbl_pct=NULL;
+        G.dl_lbl_detail=NULL;
+        G.dl_lbl_title=NULL;
+        G.dl_lbl_sub=NULL;
+    }
+    G.downloading=false;
+}
+
+void show_dl_overlay_locked(const char* ep_title, const char* slug, int season, int ep_num){
+    // No longer used - external downloader has its own UI
+    // Keep as stub that shows simple status
+    (void)ep_title; (void)slug; (void)season; (void)ep_num;
+    G.downloading=true;
+}
+
+void update_dl_overlay_ui(){
+    // No internal download progress anymore
+}
+
+bool dl_ep_raw(EpInfo* ep){
+    (void)ep;
+    return false;
+}
+
+bool dl_ep(EpInfo* ep){
+    (void)ep;
+    return false;
+}
+
+void dl_task(void* arg){
+    (void)arg;
+    G.downloading=false;
+    G.dl_handle=NULL;
+    vTaskDelete(NULL);
+}