feat(DM): instant cache load, manual refresh, full-screen season selector, season persistence fix

- Cache loads synchronously on onShow for instant startup, no auto fetch
- Added manual Refresh button, removed auto refresh that blocked UI
- Made UI non-blocking during fetch (episode list usable while fetching)
- Fixed season loss after player/downloader by preserving season in static s_preserved_season surviving G memset
- Season selector now full-screen overlay parented to lv_scr_act()
- Removed DL Missing batch button and bottom help/status label per UX request
- load_state no longer overwrites browsing season with last_slug season

Apps/DiscoveryMountain/main/Source/app_context.h

@@ -97,8 +97,15 @@ typedef struct {
     int dl_missing_total;
     int dl_missing_done;
     int dl_missing_failed;
+
+    // Refresh button reference
+    lv_obj_t *btn_refresh;
 } AppCtx;
 
+void trigger_manual_refresh(void);
+void dm_preserve_season(int season);
+int dm_consume_preserved_season(void);
+
 extern AppCtx G;
 
 #ifdef __cplusplus