refactor(shared): re-skin ThemeToggle onto SegmentedControl (§6 tokens)

Adds theme_segment_light/dark/label i18n keys (de/en/es).

Refs #857
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -32,6 +32,9 @@
 	"layout_menu_close": "Menü schließen",
 	"theme_toggle_to_light": "Zu hellem Design wechseln",
 	"theme_toggle_to_dark": "Zu dunklem Design wechseln",
+	"theme_toggle_label": "Farbschema",
+	"theme_segment_light": "Hell",
+	"theme_segment_dark": "Dunkel",
 	"btn_save": "Speichern",
 	"btn_cancel": "Abbrechen",
 	"btn_confirm": "Bestätigen",