filedialog: add open mode (no overwrite prompt) and use it for 'o'

The picker is built for saving, so it asks 'file exists — overwrite?' on
confirm. That's wrong when opening a script (you want an existing file). Add a
confirm_overwrite flag (default true; FileDialogState + OpenFileDialog param);
ConfirmFileDialog only prompts when it's set. The dashboard 'o' shortcut now
opens the dialog with confirm_overwrite=false. save/export keep the guard.

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

src/tui/screen_filedialog.cpp

@@ -54,11 +54,13 @@ void Tui::OpenFileDialog(std::string title,
                          std::string persist_key,
                          std::string default_filename,
                          std::vector<FilenameFilter> filters,
-                         std::function<void(const std::string &)> on_confirm) {
+                         std::function<void(const std::string &)> on_confirm,
+                         bool confirm_overwrite) {
     file_dialog.title       = std::move(title);
     file_dialog.persist_key = std::move(persist_key);
     file_dialog.on_confirm  = std::move(on_confirm);
     file_dialog.filters     = std::move(filters);
+    file_dialog.confirm_overwrite = confirm_overwrite;
     file_dialog.filter_labels.clear();
     for (const auto &f : file_dialog.filters)
         file_dialog.filter_labels.push_back(f.label);
@@ -122,7 +124,7 @@ void Tui::ConfirmFileDialog() {
     // Overwrite guard: if the file already exists, ask before letting
     // the action proceed. Esc / No cancels; Yes runs the action.
     std::error_code ec;
-    if (fs::exists(full, ec) && !ec) {
+    if (file_dialog.confirm_overwrite && fs::exists(full, ec) && !ec) {
         ShowConfirm("File '" + full.string() + "' already exists.\n"
                     "Overwrite?",
                     invoke);

src/tui/tui.cpp

@@ -106,7 +106,8 @@ void Tui::Run() {
                                "dashboard.source", "", {},
                                [this](const std::string &path) {
                                    Dispatch("source " + path);
-                               });
+                               },
+                               /*confirm_overwrite=*/false);  // opening, not saving
                 return true;
             }
             if (e == Event::Character("s")) {   // save the system snapshot

src/tui/tui.hpp

@@ -149,6 +149,7 @@ private:
         std::vector<std::string> filter_labels;  ///< parallel to `filters`
         int filter_idx = 0;
         std::function<void(const std::string &)> on_confirm;
+        bool confirm_overwrite = true;  ///< false in "open" mode — skip the overwrite prompt.
     };
     FileDialogState file_dialog;
 
@@ -272,11 +273,14 @@ private:
     // dir + filename are stored (one tiny file per key under the user
     // data directory). `on_confirm` runs when the user presses Enter on
     // the action button — it receives the absolute path the user picked.
+    // `confirm_overwrite = false` puts the dialog in "open" mode: it skips the
+    // "file exists — overwrite?" prompt (you *want* an existing file to open).
     void OpenFileDialog(std::string title,
                         std::string persist_key,
                         std::string default_filename,
                         std::vector<FilenameFilter> filters,
-                        std::function<void(const std::string &)> on_confirm);
+                        std::function<void(const std::string &)> on_confirm,
+                        bool confirm_overwrite = true);
     void ConfirmFileDialog();
     ftxui::Component BuildSignalTypeModal();
     ftxui::Component BuildPaletteModal();