dashboard: o/s shortcuts to open a script and save via a file dialog

Two new dashboard keys, mirroring the existing x=export: 'o' opens a file
picker to run a .essim script (-> source <path>), 's' opens a file picker to
write a system snapshot (-> save <path>, only when a system is loaded). Both
use OpenFileDialog with a per-key persisted last directory; the global
CatchEvent already yields to the file-dialog modal while it is open. Dashboard
help hints updated.

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

src/tui/screen_dashboard.cpp

@@ -309,6 +309,8 @@ Component Tui::BuildDashboardScreen() {
           {"a",      "analyze (verify + groups)"},
           {"h",      "help screen"},
           {"x",      "export"},
+          {"o",      "open/run a script"},
+          {"s",      "save system"},
           {"PgUp",   "scroll up"},
           {"PgDn",   "scroll down"},
           {"Home",   "scroll top"},

src/tui/tui.cpp

@@ -101,6 +101,23 @@ void Tui::Run() {
             if (e == Event::Character("a")) { screen_idx = 5;       return true; }
             if (e == Event::Character("h")) { screen_idx = 6;       return true; }
             if (e == Event::Character("x")) { Dispatch("export");   return true; }
+            if (e == Event::Character("o")) {   // open / run a script
+                OpenFileDialog("Open script — run a .essim file",
+                               "dashboard.source", "", {},
+                               [this](const std::string &path) {
+                                   Dispatch("source " + path);
+                               });
+                return true;
+            }
+            if (e == Event::Character("s")) {   // save the system snapshot
+                if (sys)
+                    OpenFileDialog("Save system snapshot",
+                                   "dashboard.save", "system.essim", {},
+                                   [this](const std::string &path) {
+                                       Dispatch("save " + path);
+                                   });
+                return true;
+            }
             return false;
 
         case 3:  // explore