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