Implement REQ-BLD-COPY-CONFIG-FEEDBACK: copy-settings visual feedback

Add on-screen feedback for the Shift copy-settings gesture, driven by a
single configurable color (visuals.toml [overlays].copy_config):
- eligible-target tint over every same-type building/site while a config
  is cached (source included), shown only while a paste is possible;
- a brief selection-style outline flash on copy (source) and paste (target).

Flashes use a wall-clock countdown (frame delta), so they play for a fixed
0.3 s regardless of game speed and while paused, and fade independently per
building. Extract the footprint-rect resolution shared by the selection
highlight and the new feedback into footprintWidgetRect(). Pure rendering;
no simulation or selection state touched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VGNNLeFWhVzvxkK9qVXP2K

bin/app/data/config/visuals.toml

@@ -348,6 +348,7 @@ demolish_tint  = "#ff000033"   # demolish-mode hover tint
 selection_rect = "#00ff00"     # box-drag selection rectangle (REQ-UI-MULTI-SELECT)
 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)
 
 # -----------------------------------------------------------------------------
 # Schematic-drop toasts (REQ-UI-SCHEMATIC-TOAST)