Wire main menu Options/Quit through the cutscene, fix locale crashes

Options and Quit now route through main_menu.jsonc markers the same
way New Game does, with a new QUIT_GAME cutscene item type ending the
process directly once quit is confirmed via MODAL_OPTIONS_MARKERS
(whose "message" field is now optional, defaulting to "" like "title").

Fixes two crashes surfaced along the way:
- uiModalLocalize asserted on an empty locale key instead of treating
  it as "nothing to show" - hit as soon as a modal omitted a title or
  message.
- assetLocaleGetString's new empty-message-ID guard broke the PO
  format's own header-read convention (messageId "") used internally
  by assetLocaleLoaderAsync, crashing on every boot. Extracted the
  shared lookup into assetLocaleGetStringLookup and added a dedicated
  assetLocaleGetHeader for that internal read, leaving
  assetLocaleGetString's validation untouched for every real caller.

Also replaced every raw assetLocaleGetString/WithVA call across the UI
with the locale/localemanager.h helpers (adding localeManagerGetString
for the plain, non-formatting case), so app code no longer bypasses
them.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

assets/cutscenes/main_menu.jsonc

@@ -1,9 +1,20 @@
 {
   "items": [
+    {
+      "type": "MARKER",
+      "name": "MAIN_MENU_IDLE"
+    },
     {
       "type": "UI_SHOW",
       "name": "main_menu"
     },
+
+    // Options button goes here - no options screen yet, so this is just
+    // an alias landing right back on the idle loop below.
+    {
+      "type": "MARKER",
+      "name": "OPTIONS"
+    },
     {
       "type": "IDLE"
     },
@@ -88,6 +99,35 @@
     {
       "type": "MARKER",
       "name": "LOADED"
+    },
+
+    // Quit Game button goes here. Cancelling jumps back to
+    // MAIN_MENU_IDLE; confirming falls through to QUIT_GAME, which
+    // ends the process directly.
+    {
+      "type": "MARKER",
+      "name": "QUIT"
+    },
+    {
+      "type": "MODAL_OPTIONS_MARKERS",
+      "title": "main_menu.quit_confirm",
+      "options": [
+        {
+          "text": "ui.confirm.confirm",
+          "marker": "QUIT_CONFIRMED"
+        },
+        {
+          "text": "ui.confirm.cancel",
+          "marker": "MAIN_MENU_IDLE"
+        }
+      ]
+    },
+    {
+      "type": "MARKER",
+      "name": "QUIT_CONFIRMED"
+    },
+    {
+      "type": "QUIT_GAME"
     }
   ]
 }