tui: show the Computing... progress as a real Modal (was invisible)

The Renderer-overlay approach to the global progress box didn't render. Use a
proper Modal like the palette / file dialog, driven by a plain bool
'computing_open' raised when a source starts and lowered when it ends or
aborts. The tick handler stays ahead of the modal guard, so the script keeps
running (and the screen behind it keeps updating) while the modal is shown;
computing_open is also added to the guard so stray keys are ignored mid-load.
The console screen's own Computing block was already removed, so no duplicate.

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

src/tui/shell.cpp

@@ -299,6 +299,7 @@ void Tui::Source(const std::string &filename) {
     source_origin_screen = screen_idx;  // a sourced line that leaves this screen aborts
     in_source = true;
     loading = true;
+    computing_open = true;  // raise the global "Computing…" progress modal
 
     if (!screen_ptr) {
         // Headless fallback (e.g. tests): drain synchronously.
@@ -352,6 +353,7 @@ void Tui::ProcessNextSourceLine() {
                   + " is interactive (would open a screen) — aborting.");
             screen_idx = source_origin_screen;
             loading.store(false);
+            computing_open = false;
             tick_in_flight.store(false);
             in_source = loading_prev_in_source;
             return;
@@ -364,6 +366,7 @@ void Tui::ProcessNextSourceLine() {
     Print("source: " + loading_filename
           + "  (" + std::to_string(loading_executed) + " line(s))");
     loading.store(false);
+    computing_open = false;
     tick_in_flight.store(false);
     in_source = loading_prev_in_source;
 }

src/tui/tui.cpp

@@ -59,31 +59,31 @@ void Tui::Run() {
     auto with_error   = with_confirm
                       | Modal(BuildErrorModal(), &error_open);
 
-    // Global "Computing…" overlay while a script loads — visible on any screen
-    // (not just the console), so opening a script from the dashboard shows
-    // progress instead of looking frozen.
-    auto with_loading = Renderer(with_error, [this, with_error] {
-        Element base = with_error->Render();
-        if (!loading) return base;
+    // Global "Computing…" progress modal while a script loads — a proper Modal
+    // (like the palette / file dialog), so it shows on any screen, e.g. when a
+    // script is opened from the dashboard. The Renderer re-reads the live
+    // progress every frame.
+    auto computing_modal = Renderer([this] {
         std::string progress = std::to_string(loading_executed) + " / "
                              + std::to_string((int)loading_lines.size()) + " lines";
-        Element modal = vbox({
-                            text(" Computing… ") | bold | center,
-                            separator(),
-                            text(loading_filename) | center,
-                            text(progress) | center,
-                        }) | borderDouble | size(WIDTH, GREATER_THAN, 40);
-        return dbox({base, modal | center});
+        return vbox({
+                   text(" Computing… ") | bold | center,
+                   separator(),
+                   text(loading_filename) | center,
+                   text(progress) | center,
+               }) | borderDouble | size(WIDTH, GREATER_THAN, 40);
     });
+    auto with_loading = with_error | Modal(computing_modal, &computing_open);
 
     auto root = CatchEvent(with_loading, [this](Event e) {
         // Source ticks drive the line-by-line loader; handle them on ANY screen
-        // so `source` works from the dashboard, not only the console.
+        // (and before the modal guard) so `source` keeps running while the
+        // Computing modal is up.
         if (e == Event::Special("\x02tick")) { ProcessNextSourceLine(); return true; }
 
         // Modals own their events while open. Error modal sits on top.
         if (error_open || confirm_open || palette_open
-            || sigtype_dialog_open || file_dialog.open) return false;
+            || sigtype_dialog_open || file_dialog.open || computing_open) return false;
 
         // Ctrl-P opens the palette from any screen.
         if (e == Event::CtrlP) { OpenPalette(); return true; }

src/tui/tui.hpp

@@ -104,6 +104,7 @@ class Tui {
     int loading_lineno;
     bool loading_prev_in_source;
     int source_origin_screen = 0;  ///< screen a `source` started from; a sourced line that navigates away (opens an interactive screen) aborts it.
+    bool computing_open = false;   ///< drives the global "Computing…" progress modal while a script loads.
     ftxui::ScreenInteractive *screen_ptr;  ///< set in Run() so Source() can post events.
 
     // ---- Dashboard scroll state (0 = top; grows as the user scrolls down) ----