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/en.json

@@ -32,6 +32,9 @@
 	"layout_menu_close": "Close menu",
 	"theme_toggle_to_light": "Switch to light mode",
 	"theme_toggle_to_dark": "Switch to dark mode",
+	"theme_toggle_label": "Color scheme",
+	"theme_segment_light": "Light",
+	"theme_segment_dark": "Dark",
 	"btn_save": "Save",
 	"btn_cancel": "Cancel",
 	"btn_confirm": "Confirm",