fix(ui): stop global shortcuts auto-repeating; skip invalid skins

The global keyboard shortcuts tested keys without disabling key-repeat,
so holding one down fired it every frame: the theme cycle, balance and
market layout cycles, the effects and background toggles, the settings
and sidebar-collapse keys all re-triggered on a held key. Cycling themes
also stepped onto skins that had failed to load and wrote the broken
pick into settings.

- Every one-shot shortcut now checks for a fresh key press only, so a
  held key fires once.
- Theme cycling skips over invalid skins and only applies, persists, and
  shows the toast when a skin actually loads; if none can load, settings
  are left untouched and no toast appears.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/balance_tab.cpp

@@ -172,8 +172,8 @@ void RenderBalanceTab(App* app)
     // Left/Right arrows: cycle through enabled balance layouts
     // (skip when Ctrl is held — Ctrl+Arrow cycles themes instead)
     if (app->settings() && !ImGui::GetIO().WantTextInput && !ImGui::GetIO().KeyCtrl) {
-        bool cycleUp   = ImGui::IsKeyPressed(ImGuiKey_LeftArrow);
-        bool cycleDown = ImGui::IsKeyPressed(ImGuiKey_RightArrow);
+        bool cycleUp   = ImGui::IsKeyPressed(ImGuiKey_LeftArrow, false);
+        bool cycleDown = ImGui::IsKeyPressed(ImGuiKey_RightArrow, false);
         if (cycleUp || cycleDown) {
             const auto& layouts = GetBalanceLayouts();
             // Build list of enabled layout IDs