Replace gtk_dialog_run with async signal handlers (13/20 instances)

[Copilot]

gtk_dialog_run blocks the UI thread and is removed in GTK4. This PR converts dialog handling to async patterns using GTK signal handlers.

Changes

Converted to async (13/20 instances):

• Simple notifications (pop_info, pop_warning, AboutDialog, error dialogs): Connect response signal to gtk_widget_destroy, show dialog non-blocking
• File choosers (choose_file_with_preview, PickEnclosure, OnSaveImage, ShareFile pickers): New *_async() variants accept std::function<void(T*)> callbacks
• Modal dialogs (ShareFile, DataSettings): Response handlers check action (OK/APPLY/CANCEL), destroy or keep open as needed

Pattern:

// Before
gchar* filename = choose_file_with_preview(title, parent);
if (filename) { process(filename); }

// After  
choose_file_with_preview_async(title, parent, [](gchar* filename) {
  if (filename) { process(filename); }
});

Remaining (7/20 instances):

Functions returning values synchronously (pop_request_quit, pop_obtain_shared_passwd, pop_password_settings, pop_save_path, validation loops in RevisePal, DetectPal). These require API changes at call sites.

Technical Details

• Use g_signal_connect_data with custom data structs for memory management
• Lambda captures with [this] or [=] for context preservation
• Added <functional> header to files using std::function

Original prompt

---

This section details on the original issue you should resolve

<issue_title>gtk_dialog_run should be removed</issue_title>
<issue_description>1. Gtk4 removed it
2. it will block the UI thread, which cause other events unresponsible.
3. hard to use in unittest</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes gtk_dialog_run should be removed #718

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[github-actions]

Test Results

0 tests   - 69   0 ✅  - 69   0s ⏱️ -3s
0 suites  - 32   0 💤 ± 0 
0 files    -  1   0 ❌ ± 0 

Results for commit bbd167a. ± Comparison against base commit 04a6534.