Move uimodal to ui/widget, make main menu its own scene

uimodal.c/.h now live in ui/widget alongside the other widgets rather
than ui/frame. The main menu now has a real scene (scene/mainmenu)
whose init just calls the new uiMainMenuOpen(), replacing the
SCENE.current-polling uiMainMenuUpdate with an explicit open call like
uiGameMenuOpen/uiBackpackOpen. Also renames the ui.main_menu.* locale
keys to main_menu.* and groups them under a Main Menu Scene heading in
en_US.po, alongside the existing Initial Scene keys.

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

assets/locale/en_US.po

@@ -5,6 +5,7 @@ msgstr ""
 "Content-Type: text/plain; charset=UTF-8\n"
 "Plural-Forms: nplurals=4; plural=(n==1 ? 0 : n==2 ? 1 : (n<7 ? 2 : 3));\n"
 
+# Initial Scene
 msgid "initial.checking_save.title"
 msgstr "Checking for save data"
 
@@ -23,13 +24,18 @@ msgstr "Try again"
 msgid "initial.no_device.continue"
 msgstr "Continue without saving"
 
+# Main Menu Scene
+msgid "main_menu.new_game"
+msgstr "New Game"
 
+msgid "main_menu.load_game"
+msgstr "Load Game"
 
+msgid "main_menu.options"
+msgstr "Options"
 
-
-
-
-
+msgid "main_menu.quit"
+msgstr "Quit Game"
 
 #: ui/menu.c:10
 msgid "ui.title"
@@ -141,22 +147,6 @@ msgstr "Yes"
 msgid "ui.initial.create_save.no"
 msgstr "No"
 
-#: src/dusk/ui/frame/mainmenu/uimainmenu.c
-msgid "ui.main_menu.new_game"
-msgstr "New Game"
-
-#: src/dusk/ui/frame/mainmenu/uimainmenu.c
-msgid "ui.main_menu.load_game"
-msgstr "Load Game"
-
-#: src/dusk/ui/frame/mainmenu/uimainmenu.c
-msgid "ui.main_menu.options"
-msgstr "Options"
-
-#: src/dusk/ui/frame/mainmenu/uimainmenu.c
-msgid "ui.main_menu.quit"
-msgstr "Quit Game"
-
 #: src/dusk/ui/frame/uiconfirm.c
 msgid "ui.confirm.confirm"
 msgstr "Confirm"