Implement REQ-UI-MODAL-DIM: dim game behind modals

Add a window-wide semi-transparent black scrim drawn over the entire
game window (header bar, game world view, side panel column) while any
auto-pausing modal or end-game screen is open, behind that modal.

- New ModalDimOverlay widget: a full-window, mouse-transparent child of
  MainWindow that paints the dim color. Visibility is reference-counted
  (pushModal/popModal) so nested or back-to-back modals show a single
  dim, not stacked layers; a ModalDimScope RAII guard drives it.
- Wire a guard around every modal opened from MainWindow: schematic
  choice, escape menu, layout dialog, recipe/schematic selection
  (spanning the auto-opened layout dialog), game-over, and win. The
  nested Create Blueprint dialog is covered by the layout-dialog dim.
- Config-driven color via visuals.toml [overlays].modal_dim, following
  the existing OverlayVisuals/parseColor pattern; refreshed on the
  Restart config-reload paths (REQ-CFG-RELOAD).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps

bin/app/data/config/visuals.toml

@@ -350,6 +350,7 @@ tile_highlight = "#ffffff22"   # tile under cursor
 selected_outline = "#ffff00"   # outline drawn around currently-selected building(s)
 copy_config    = "#33ccff66"   # copy-settings eligible-target tint + copy/paste flash (REQ-BLD-COPY-CONFIG-FEEDBACK)
 locked_asteroid = "#0000007f"  # tint over the asteroid left of the buildable edge (not yet unlocked by expansion)
+modal_dim      = "#00000099"   # semi-transparent black dim behind modal dialogs/menus (REQ-UI-MODAL-DIM)
 
 # -----------------------------------------------------------------------------
 # Schematic-drop toasts (REQ-UI-SCHEMATIC-TOAST)