Render produced-item icons in recipe dialog and game world

Implements REQ-UI-ITEM-ICON and REQ-UI-RECIPE-ICON. Adds an optional
recipes.toml `icon` field (an item id; defaults to the recipe's first
output). New ItemIconCache loads per-item SVGs from icons/items and
caches pixmaps per target pixel size, shared by:
- the Miner/Assembler recipe-selection dialog, which now shows icon-only
  option buttons (name-caption fallback when no icon file exists), and
- GameWorldView belt/port item rendering via a shared drawWorldItem
  helper (colored-square fallback preserved).

Shipyard schematic dialog is unchanged. No icon art is shipped; every
path falls back cleanly when an item has no SVG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/lib/config/RecipesConfig.h

@@ -32,6 +32,10 @@ struct RecipeDef
     std::vector<RecipeIngredient> inputs;
     std::vector<RecipeOutput> outputs;
     double durationSeconds;
+    // Optional id of the item whose icon represents this recipe in the recipe-
+    // selection dialog (REQ-UI-RECIPE-ICON). When unset, the first output item is
+    // used. A missing icon file for that item is not an error (REQ-UI-ITEM-ICON).
+    std::optional<std::string> icon;
     // Assembler only. When true, this recipe is available from game start
     // regardless of the implicit item graph — used for base recipes that no
     // schematic's materials reach (e.g. building blocks). See REQ-LOCK-IMPLICIT.