open_file: file associations, default editor and 'Open with' dialog

- New open_file IPC message: resolves [open_with] extension mapping,
  falls back to default_editor for known text formats, otherwise opens
  an 'Öppna med' dialog where the user types a command (live preview
  of the full command line, path tail-truncated so the extension stays
  visible) and chooses open-once or save-as-default for the extension
  (persisted to [open_with] in config.toml)
- default_editor configurable in the settings dialog (new Editor row
  with inline text editing) and in config.toml; '{}' in commands is
  replaced with the shell-quoted path
- Documented in SOCKET_API.md; integration suite covers dialog →
  save-association → direct open (38/38)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main.rs

@@ -597,6 +597,9 @@ fn handle_ipc_event(
                                     format!("[Popup: {}]", &message[..message.len().min(20)])
                                 }
                                 WindowContent::Settings { .. } => "[Inställningar]".to_string(),
+                                WindowContent::OpenWith { path, .. } => {
+                                    format!("[Öppna: {}]", path)
+                                }
                             };
                             Some(WindowInfo {
                                 id: w.id,
@@ -615,6 +618,14 @@ fn handle_ipc_event(
                 ClientMessage::CloseWindow { window_id, .. } => {
                     app.close_window_pub(window_id);
                 }
+                ClientMessage::OpenFile { path, request_id } => {
+                    let window_id = app.open_file(&path);
+                    app.ipc_out.push(AppIpcOut::WindowOpened {
+                        client_id,
+                        request_id,
+                        window_id,
+                    });
+                }
                 ClientMessage::SetBackground { path, save, request_id } => {
                     app.config.background_image = path.clone();
                     if save {