Megamap: the zoom button zooms on the hero and follows him

The zoom button never set the zoom point, so it zoomed on the last
magnifier point (or 0,0). While the button's zoom is on, the point is put on
the hero's map pixel every engine tick; the held mouse button still zooms on
the mouse. game::UiWindow / UiWindowShown shared from windows.h.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/todo.md

@@ -230,14 +230,6 @@ Request: NPCs related to a quest are drawn white on the minimaps and always
 turn towards the player; once their quest is finished they should stop
 turning and be drawn blue (like other NPCs).
 
-## Megamap: zoom not centred on the hero (2026-10-25)
-
-Reported: the zoomed view does not start centred on the hero. The game zooms
-on the large map's zoom point +0xc0438 (the mouse position, updated while the
-mouse is outside the zoom buttons +0xc0538; 0x6ca47b) plus the scroll. Open:
-how the user zoomed (buttons / key / click); then zoom on the hero for
-buttons and keys.
-
 ## Inventory drawn without its dark backgrounds (2026-10-25, intermittent)
 
 Reported twice: on opening the inventory the item list had no dark