Remove dead always-true runnable action field

runnable was hardcoded true on every action,
and only loaded+enabled capabilities ever
reach the registry, so every JS === false
branch was unreachable. Drop the field, its
JSON, the guards, and the orphan .disabled
CSS rule.

src/slic3r/GUI/ActionRegistry.cpp

@@ -91,7 +91,6 @@ bool ActionRegistry::make_action(const std::string& plugin_key, const std::strin
     out.pkg        = package_name_for(plugin_key);
     out.plugin_key = plugin_key;
     out.capability = capability;
-    out.runnable   = true;
     seed_state(out);
     return true;
 }
@@ -295,7 +294,6 @@ nlohmann::json ActionRegistry::snapshot() const
         actions.push_back({{"id", a->id},
                            {"title", a->title},
                            {"pkg", a->pkg},
-                           {"runnable", a->runnable},
                            {"shortcut", ""}});
     // why: favourites is the ORDERED pin list - it must come from favourite_actions
     // as stored, not be re-derived from the frecency-sorted actions (that would

src/slic3r/GUI/ActionRegistry.hpp

@@ -26,7 +26,6 @@ struct AppAction
     std::string pkg;          // owning package display name (row eyebrow)
     std::string plugin_key;   // weak handle: owning package
     std::string capability;   // weak handle: capability name
-    bool        runnable = true;
 
     // seeded from AppConfig for the snapshot / sort:
     bool        favourite = false;