feat(frontend): regroup settings dialog, icon renderer tiles

Settings splits into labelled sections — renderer, appearance, wallpaper
and CSS, notifications, auth, backup — each with an icon header. Renderer
options gain a per-style glyph and accent-highlighted selection; theme
becomes a segmented Dark/Light control; the dialog widens to max-w-xl.

Renderer switching itself verified working end-to-end (live switch +
persistence across reload); unchanged code path since v2.0.0.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/frontend/src/i18n/cs.ts

@@ -109,6 +109,7 @@ export const cs: Partial<Record<Key, string>> = {
 	"settings.testNotify": "Odeslat testovací notifikaci",
 	"settings.wallpaperPh": "https://… — prázdné = žádná",
 	"settings.importFailed": "Import selhal",
+	"settings.backup": "Záloha a import",
 
 	"service.editTitle": "Upravit službu",
 	"service.newTitle": "Přidat službu",