float the selection panel over the game world instead of a side column

Implements REQ-UI-SELECTION-PANEL and the full-width REQ-UI-WORLD-SIZE /
REQ-UI-HEADER: MainWindow drops the 25% side column and its 75/25 math, so
the header bar and world view span the window, and the panel joins the
build button bar as a widget floating over the world.

The panel follows the bar's pattern -- an opaque sibling built after the
world view, so it sits above the vignettes, below the dim overlay, and
swallows the mouse events that would otherwise reach the world. It sizes
itself to its content within a band the window hands it (the world view
less the bar's strip, so the bar never has to move for it), right-aligned
and centered in that band, and scrolls once the content outgrows it. Its
content moved into a scroll area for that; the width is capped at 320 px
because the wrapped labels and the splitter filter lists have no natural
width of their own. With nothing selected the panel now hides entirely
rather than showing an empty box (REQ-UI-EMPTY-SELECTION).

Two defects that content sizing exposed: buildEmpty() left the selected
ids behind when the building vanished under the panel, which would have
held an empty panel on screen, and buildMulti() let a shipyard's layout
preview survive from a previous single selection (REQ-UI-MULTI-SELECTION).

Renames SelectedBuildingPanel to SelectionPanel throughout, matching the
requirements: the panel has long shown ships, stations, and debris too.

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

src/ui/BuildButtonBar.h

@@ -51,6 +51,11 @@ public:
     // re-center later driven by an unlock needs no second call from the owner.
     void anchorTo(const QRect& worldViewRect);
 
+    // Height of the strip the bar occupies along the bottom of the world view: its own
+    // height plus the margin below it. The selection panel keeps out of this strip, and
+    // the bar never moves for the panel in return (REQ-UI-BUILD-BAR).
+    int getStripHeightPx() const;
+
     void clearActiveButton();
 
 private: