float the build buttons as a bar over the game world

Implements the REQ-UI-BUILD-BAR rewrite: the build buttons leave the side
panel column for a widget floating at the bottom center of the game world
view, as one non-wrapping row. BuildButtonGrid is renamed BuildButtonBar and
its QGridLayout becomes a QHBoxLayout; the side panel column is left with two
equal-height panels.

Buttons become icon-only. Each face is composed into a single pixmap per
QIcon mode - hotkey badge in the top-left corner, chip icon centered, cost
and block icon below - because a QPushButton holds only one icon. That
replaces the custom-painted BuildButton and lets Qt grey an unaffordable
button by swapping the pixmap. The building name moves into the tooltip,
which now always leads with it, and a missing chip SVG falls back to the name
in the chip's place.

The badge labels come from InputMapper, searched out of the same table the
key handler uses so a badge cannot claim a key that does nothing. The
Deconstruct button has no cost, so it shows its name there instead, and sits
last behind a gap.

Parenting the bar to MainWindow makes creation order the stacking order: it
lands above the world view and its vignettes and below the modal dim, and Qt
routes mouse events to it rather than the world, which is the whole input
clause of REQ-UI-BUILD-BAR at no cost.

Requirements are amended for the two things the mockup added: the hotkey
badge and the Deconstruct caption.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JcReq7hVk4KUPhTDKWAG7K

src/ui/GameWorldView.cpp

@@ -332,7 +332,7 @@ void GameWorldView::onFrame()
         }
 
         // Unlocked building set changes only after a drop is applied or on Restart
-        // (REQ-LOCK-BUILDING); the build grid rebuilds its visible buttons.
+        // (REQ-LOCK-BUILDING); the build bar rebuilds its visible buttons.
         int newUnlockedBuildingCount = 0;
         for (const BuildingDef& def : m_sim->getConfig().buildings.buildings)
         {