yoda: add "Always use this app" toggle to OpenWith dialog

The 'Open with...' dialog let you pick an app but never remembered your
choice — you'd see the same dialog again next time. The infrastructure
was already there (MimeAppCache::set_default writes to mimeapps.list),
just never wired to the UI.

Adds a toggler below the app list labelled 'Always use this app for this
file type' (EN) / 'Toujours utiliser cette application pour ce type de
fichier' (FR). When enabled, after spawning the selected app, the
default handler for the file's mime type is persisted via
self.mime_app_cache.set_default(mime, app.id).

Implementation:
- DialogPage::OpenWith gains a set_default: bool field (defaulted false)
- Message::OpenWithToggleDefault(bool) + handler mutates the dialog state
- DialogComplete handler for OpenWith calls set_default after a clean
  spawn when the flag is set
- Dialog rendering adds a .control(widget::row) with label + toggler,
  between the scrollable list and the action buttons
- i18n strings added: en/fr open-with-set-default

i18n/en/cosmic_files.ftl

@@ -96,6 +96,7 @@ save-file = Save file
 
 ## Open With Dialog
 open-with-title = How do you want to open "{$name}"?
+open-with-set-default = Always use this app for this file type
 browse-store = Browse {$store}
 other-apps = Other applications
 related-apps = Related applications

i18n/fr/cosmic_files.ftl

@@ -92,6 +92,7 @@ save-file = Enregistrer fichier
 ## Open With Dialog
 
 open-with-title = Comment souhaitez-vous ouvrir "{ $name }" ?
+open-with-set-default = Toujours utiliser cette application pour ce type de fichier
 browse-store = Parcourir { $store }
 
 ## Permanently delete Dialog

src/app.rs

@@ -407,6 +407,7 @@ pub enum Message {
     OpenWithBrowse,
     OpenWithDialog(Option<Entity>),
     OpenWithSelection(usize),
+    OpenWithToggleDefault(bool),
     #[cfg(all(feature = "wayland", feature = "desktop-applet"))]
     Overlap(window::Id, OverlapNotifyEvent),
     Paste(Option<Entity>),
@@ -577,6 +578,9 @@ pub enum DialogPage {
         mime: mime_guess::Mime,
         selected: usize,
         store_opt: Option<MimeApp>,
+        /// When true, the chosen app is written to mimeapps.list as the
+        /// default handler for `mime` after it spawns.
+        set_default: bool,
     },
     PermanentlyDelete {
         paths: Box<[PathBuf]>,
@@ -3232,6 +3236,7 @@ impl Application for App {
                             path,
                             mime,
                             selected,
+                            set_default,
                             ..
                         } => {
                             let available_apps = self.get_apps_for_mime(&mime);
@@ -3250,6 +3255,11 @@ impl Application for App {
                                                     None,
                                                 );
                                             }
+                                            // Yoda: persist as default if the user asked for it.
+                                            if set_default {
+                                                self.mime_app_cache
+                                                    .set_default(mime.clone(), app.id.clone());
+                                            }
                                         }
                                         Err(err) => {
                                             log::warn!(
@@ -3872,6 +3882,7 @@ impl Application for App {
                                     .and_then(|mime| {
                                         self.mime_app_cache.get(&mime).first().cloned()
                                     }),
+                                set_default: false,
                             },
                             Some(CONFIRM_OPEN_WITH_BUTTON_ID.clone()),
                         );
@@ -3883,6 +3894,13 @@ impl Application for App {
                     *selected = index;
                 }
             }
+            Message::OpenWithToggleDefault(enabled) => {
+                if let Some(DialogPage::OpenWith { set_default, .. }) =
+                    self.dialog_pages.front_mut()
+                {
+                    *set_default = enabled;
+                }
+            }
             Message::Paste(entity_opt) => {
                 let entity = entity_opt.unwrap_or_else(|| self.tab_model.active());
                 if let Some(tab) = self.tab_model.data_mut::<Tab>(entity)
@@ -5110,6 +5128,7 @@ impl Application for App {
                                             .and_then(|mime| {
                                                 self.mime_app_cache.get(&mime).first().cloned()
                                             }),
+                                        set_default: false,
                                     },
                                     None,
                                 );
@@ -5953,7 +5972,7 @@ impl Application for App {
                 mime,
                 selected,
                 store_opt,
-                ..
+                set_default,
             } => {
                 let name = match path.file_name() {
                     Some(file_name) => file_name.to_str(),
@@ -6038,7 +6057,21 @@ impl Application for App {
                         } else {
                             Length::Shrink
                         }
-                    }));
+                    }))
+                    // Yoda: let the user make this choice stick. A plain row
+                    // instead of settings::item::builder because the latter
+                    // returns a section Item, not an Element usable in .control().
+                    .control(
+                        widget::row::with_children([
+                            widget::text::body(fl!("open-with-set-default")).into(),
+                            widget::space::horizontal().into(),
+                            widget::toggler(*set_default)
+                                .on_toggle(Message::OpenWithToggleDefault)
+                                .into(),
+                        ])
+                        .spacing(space_s)
+                        .align_y(Alignment::Center),
+                    );
 
                 if let Some(app) = store_opt {
                     dialog = dialog.tertiary_action(