Rebuild uiconfirm on top of uimodal, add quit confirmation

uiconfirm no longer duplicates its own backdrop/frame/text/menu drawing
- it now just opens the shared uimodal_t with a Confirm/Cancel option
pair and translates the result back to a bool. uimodal itself gains an
optional title (NULL skips the title row entirely) so a title-less
confirm dialog doesn't reserve blank space.

Main menu's Quit option now opens a confirm dialog instead of exiting
immediately.

Drops es_MX.po/jp_JP.po locale files.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

assets/locale/en_US.po

@@ -24,6 +24,7 @@ msgstr "Try again"
 msgid "initial.no_device.continue"
 msgstr "Continue without saving"
 
+
 # Main Menu Scene
 msgid "main_menu.new_game"
 msgstr "New Game"
@@ -37,6 +38,19 @@ msgstr "Options"
 msgid "main_menu.quit"
 msgstr "Quit Game"
 
+msgid "main_menu.quit_confirm"
+msgstr "Are you sure you want to quit?"
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 #: ui/menu.c:10
 msgid "ui.title"
 msgstr ""