Re-enable save system, fix header/version stamping, handle missing media

- Fixed the actual reason saving never worked on any platform: saveWrite()
  never stamped file->header/file->version before serializing, so every
  written save file had a zeroed magic header and failed its own
  validation on the next load. Confirmed via a manual write/load round
  trip that this alone fully explains "saving doesn't work."
- Re-enabled saveInit()/saveDispose() in engine.c (previously commented out
  under "Temporarily disable save code").
- Added SAVE.available + saveIsAvailable(), refreshed by every real
  save/load/delete attempt. saveInit() no longer treats an unreachable
  save medium as fatal to booting - it logs and continues, since a missing
  memory card/stick shouldn't prevent playing.
- Hardened PSP's saveInitPSP() to actually detect a missing memory stick
  (sceIoGetstat on ms0:/) instead of assuming success, and fixed
  single-level sceIoMkdir to build the full PSP/SAVEDATA directory chain.
- Added busy-retry (CARD_ERROR_BUSY) and a not-mounted guard to Dolphin's
  live savestreamdolphin.c path, extending the same handling already
  backported into savedolphin.c.
- Added a "Save" entry to the game menu wired to saveWrite(0), showing a
  clear message on success, on failure, and when saveIsAvailable() is false.

assets/locale/en_US.po

@@ -56,6 +56,10 @@ msgstr "Items"
 msgid "ui.game_menu.settings"
 msgstr "Settings"
 
+#: src/dusk/ui/frame/game/uigamemenu.c
+msgid "ui.game_menu.save"
+msgstr "Save"
+
 msgid "item.potion.name"
 msgstr "Potion"
 

assets/locale/es_MX.po

@@ -57,6 +57,10 @@ msgstr "Objetos"
 msgid "ui.game_menu.settings"
 msgstr "Configuración"
 
+#: src/dusk/ui/frame/game/uigamemenu.c
+msgid "ui.game_menu.save"
+msgstr "Guardar"
+
 #: src/dusk/rpg/item/item.json
 msgid "item.potion.name"
 msgstr "Poción"

assets/locale/jp_JP.po

@@ -57,6 +57,10 @@ msgstr "アイテム"
 msgid "ui.game_menu.settings"
 msgstr "設定"
 
+#: src/dusk/ui/frame/game/uigamemenu.c
+msgid "ui.game_menu.save"
+msgstr "セーブ"
+
 #: src/dusk/rpg/item/item.json
 msgid "item.potion.name"
 msgstr "ポーション"