clear the selection with Q, and drop it on entering a build mode

Implements the requirements committed in 731b887.

Q becomes a three-way branch in the action table, which is the layer that owns
what an input does: ExitMode while a mode is active, the new ClearSelection
while something is selected, EnterDeconstruct otherwise. The three partition
the situations between them, so resolution stays first-match-wins over
available actions and the handler never re-derives the precedence -- which is
why ClearSelection gets its own event rather than joining ModeCancel on Q.

GameWorldView clears the selection at each of the three events that enter a
mode; those are the only ways in, whichever button or key the player used. No
clear is needed where ModeCancel falls through to deconstruct mode: Q resolves
to ClearSelection while anything is selected, so there is nothing left by then.

The Selection context's Q row reads "Clear selection", sits last as the row
that hands the context back does everywhere, and carries the exit badge
styling -- one key that backs out should look the same wherever it appears.
Requirements follow that last point in REQ-UI-CONTROLS-CARD and
REQ-UI-CONTROLS-CONTENT.

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

src/ui/ControlActionText.cpp

@@ -63,6 +63,7 @@ QString getControlActionLabel(ControlAction action, const ControlContext& contex
     case ControlAction::EnterDeconstruct:   return Strings::tr("Deconstruct mode");
     case ControlAction::CopyTemporary:      return Strings::tr("Copy to temporary blueprint");
     case ControlAction::CreateBlueprint:    return Strings::tr("Create blueprint");
+    case ControlAction::ClearSelection:     return Strings::tr("Clear selection");
     case ControlAction::Place:              return Strings::tr("Place");
     case ControlAction::ApplySettings:      return Strings::tr("Apply settings");
     case ControlAction::PlaceBeltLine:      return Strings::tr("Place belt line");