ask the icon cache for the inline block icon

The three block-icon call sites each spelled out the same expression that
ItemIconCache::getInlineIcon() now is: the bare icon at the line's text
height, null when the file is missing. They call the helper instead, and
HeaderBar::blockIcon() -- which was that expression and nothing else --
is gone along with its stale "loaded once on first use" comment.

Each site still passes its own widget font, so nothing about the sizing
changes; the rule now lives in one place rather than four.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/ui/BuildButtonBar.cpp

@@ -175,9 +175,7 @@ BuildButtonBar::BuildButtonBar(Simulation* sim, const GameConfig* config,
 
     // Block icon shown to the right of each button's cost (REQ-UI-BUILD-COST); null
     // when no building_block icon exists, in which case the cost is the bare number.
-    const QPixmap blockIcon = m_itemIcons->hasIcon(kBlockItemId)
-        ? m_itemIcons->getPixmap(kBlockItemId, QFontMetrics(font()).height())
-        : QPixmap();
+    const QPixmap blockIcon = m_itemIcons->getInlineIcon(kBlockItemId, font());
 
     for (const BuildingDef& def : config->buildings.buildings)
     {