Branding: Farben systemweit einstellbar + Bild-Upload statt nur URLs

Design-Tab (Administration → Einstellungen → Design):
- Akzentfarbe für Hell- und Dark-Mode, Markenverlauf und Eckenradius
- abgeleitete Töne (Hover, weiche Flächen, Fokusring) werden per
  color-mix aus der Grundfarbe berechnet – eine Farbe genügt
- Live-Vorschau mit Button, Badge, Chip, Logo-Kachel und Link
- Ausgabe als schlanker :root-Override über Ui::brandingStyle(), greift
  auf allen Seiten inklusive Login und Installer

Uploads (includes/Upload.php):
- Logo, Favicon, Login-Logo und Login-Hintergrund lassen sich jetzt
  hochladen; das URL-Feld bleibt als Alternative bestehen
- Whitelist nach Endung, 3-MB-Grenze, getimagesize-Prüfung für Raster,
  SVGs werden von Skripten, Event-Handlern und externen Verweisen befreit
- Zufällige Dateinamen in uploads/, dort sperrt eine .htaccess die
  Ausführung von PHP; beim Ersetzen wird die alte Datei gelöscht

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lang/en.php

@@ -220,6 +220,28 @@ return [
     // Settings
     'settings_title'    => 'Settings',
     'settings_subtitle' => 'System configuration and customization',
+    'settings_tab_branding'       => 'Design',
+    'settings_branding_intro'     => 'Colours and shapes for the whole interface – front end and administration.',
+    'settings_brand_accent'       => 'Accent colour (light)',
+    'settings_brand_accent_hint'  => 'Buttons, active navigation, links. Derived shades are calculated automatically.',
+    'settings_brand_accent_dark'  => 'Accent colour (dark mode)',
+    'settings_brand_accent_dark_hint' => 'Usually a lighter variant of the base colour for dark mode.',
+    'settings_brand_gradient_from'=> 'Brand gradient: start',
+    'settings_brand_gradient_to'  => 'Brand gradient: end',
+    'settings_brand_gradient_hint'=> 'Used for the logo tile, avatars and the voucher card.',
+    'settings_brand_radius'       => 'Corner radius',
+    'settings_brand_radius_sharp' => 'Sharp',
+    'settings_brand_radius_default'=> 'Default',
+    'settings_brand_radius_round' => 'Round',
+    'settings_brand_preview'      => 'Preview',
+    'settings_brand_link'         => 'Example link',
+    'settings_image_url_placeholder' => 'https://… or upload a file',
+    'settings_image_remove'       => 'Remove image',
+    'settings_upload_hint'        => 'PNG, JPG, WEBP, GIF or SVG – 3 MB maximum.',
+    'upload_error_generic'        => 'The file could not be uploaded.',
+    'upload_error_size'           => 'The file is too large (3 MB maximum).',
+    'upload_error_type'           => 'This file type is not supported.',
+    'upload_error_dir'            => 'The uploads/ directory is not writable.',
     'settings_tab_login'          => 'Login page',
     'settings_login_intro'        => 'Appearance and wording of the sign-in page. Empty fields fall back to the defaults.',
     'settings_login_panel'        => 'Show left image column (split screen)',
@@ -249,8 +271,8 @@ return [
     'settings_tab_password' => 'Password',
     'settings_saved'        => 'Settings saved successfully!',
     'settings_app_title'    => 'Application Title *',
-    'settings_logo_url'     => 'Logo URL',
-    'settings_favicon_url'  => 'Favicon URL',
+    'settings_logo_url'     => 'Logo',
+    'settings_favicon_url'  => 'Favicon',
     'settings_favicon_hint' => 'Browser tab icon (.ico, .png, .svg)',
     'settings_instr_header' => 'Instructions - Headline',
     'settings_instr_text'   => 'Instructions - Text',