Add save slot picker (uiselectsave/uisaveslot), fix focus stack reentrancy

uiselectsave is a Save/Load/Delete slot picker built on save/save.h's
SAVE_SLOT_COUNT: a scrolling list of framed uisaveslot_t rows (slot
number + file name, or an "Empty Slot" placeholder), navigated via the
same uimenu_t/focus-stack convention as every other menu. Load and
Delete modes add a trailing action row (switch to delete / cancel)
fixed below the scrollable area. Selecting a slot in Delete mode routes
through uiConfirmOpen before actually blanking the slot via
saveSlotInit + saveDeviceSlotWrite. uiScrollingEnsureVisible in
uiscrolling.c is now a real implementation instead of the earlier
placeholder.

Main menu collapses New Game/Load Game into a single Start Game entry
that opens uiselectsave in Load mode; backing out reopens the main
menu via uiselectsave's result callback.

Also fixes a real reentrancy bug in uiFocusPop: it invoked the popped
item's closed callback before decrementing UI_FOCUS.count, so a push
triggered from within that callback (e.g. reopening the main menu once
select-save reports no result) landed one slot past the true stack top
and got silently dropped, stranding the new item outside navigation.
Count is now decremented first.

Adds a uikeyboard placeholder widget, registered in uielementlist with
no behavior yet.

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

assets/locale/en_US.po

@@ -26,11 +26,8 @@ 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.start_game"
+msgstr "Start Game"
 
 msgid "main_menu.options"
 msgstr "Options"
@@ -42,6 +39,26 @@ msgid "main_menu.quit_confirm"
 msgstr "Are you sure you want to quit?"
 
 
+# Select Save Screen
+msgid "ui.select_save.title"
+msgstr "Select Save"
+
+msgid "ui.select_save.empty"
+msgstr "Empty Slot"
+
+msgid "ui.select_save.slot_format"
+msgstr "%s  Lv.%d  %s"
+
+msgid "ui.select_save.delete_mode"
+msgstr "Delete a Save"
+
+msgid "ui.select_save.delete_confirm"
+msgstr "Are you sure you want to delete this save?"
+
+msgid "ui.save_slot.number_format"
+msgstr "Slot %d"
+
+