feat(web): admin theme import + branding dimension hints

- Admin default-theme editor: "Theme importieren" button loads a .theme.json
  into the editor (colours only, live preview) so an imported theme can be set
  as the instance default (still confirmed via "Standard speichern").
- Branding: drop the "Standard" placeholder; show pixel-dimension hints instead
  (logo shown at max 150×40, favicon 128×128). Logo preview box now mirrors its
  real topbar footprint.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/i18n.js

@@ -69,8 +69,12 @@ const translations = {
     admin_default_theme: 'Standard-Theme',
     admin_default_theme_desc: 'Gilt für alle Nutzer ohne eigene Farbe. „Zurück" bei einer Farbe springt auf diesen Standard.',
     admin_theme_save: 'Standard speichern',
+    admin_theme_import: 'Theme importieren',
+    admin_theme_imported: '{count} Farben importiert – zum Übernehmen speichern',
     admin_theme_discard: 'Verwerfen',
     admin_theme_saved: 'Standard-Theme gespeichert',
+    admin_logo_dims: 'PNG/JPEG/WebP · Anzeige max. 150 × 40 px',
+    admin_favicon_dims: 'PNG/JPEG/WebP · 128 × 128 px (quadratisch)',
     admin_branding: 'Branding',
     admin_branding_desc: 'Eigenes Logo (oben links) und Favicon für die ganze Instanz. PNG/JPEG/WebP, max. 5 MB.',
     admin_logo: 'Logo',
@@ -462,8 +466,12 @@ const translations = {
     admin_default_theme: 'Default theme',
     admin_default_theme_desc: 'Applies to all users who have not set their own colour. A colour "Reset" returns to this default.',
     admin_theme_save: 'Save default',
+    admin_theme_import: 'Import theme',
+    admin_theme_imported: '{count} colours imported – click Save to apply',
     admin_theme_discard: 'Discard',
     admin_theme_saved: 'Default theme saved',
+    admin_logo_dims: 'PNG/JPEG/WebP · shown at max 150 × 40 px',
+    admin_favicon_dims: 'PNG/JPEG/WebP · 128 × 128 px (square)',
     admin_branding: 'Branding',
     admin_branding_desc: 'Custom logo (top-left) and favicon for the whole instance. PNG/JPEG/WebP, max 5 MB.',
     admin_logo: 'Logo',