feat: add foodsharing manager branding

Add supplied logos, favicons, PWA metadata, branded login and navigation,
and apply the Foodsharing Manager green palette across existing views.

images/foodsharing-manager-logo-assets/README.md

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+# Foodsharing Manager – Logo-Assets
+
+## Empfohlene Dateien
+
+### Klassische Webseite
+- `favicon/favicon.ico`
+- `favicon/favicon-32x32.png`
+- `mobile/apple-touch-icon-180x180.png`
+- `pwa/site.webmanifest`
+
+### PWA / Android
+- `pwa/android-chrome-192x192.png`
+- `pwa/android-chrome-512x512.png`
+- `pwa/maskable-icon-512x512.png`
+
+### Desktop-App
+- Windows: `desktop/app-icon.ico`
+- macOS: `desktop/app-icon.icns`
+- Linux / Electron / Tauri: `desktop/app-icon-512x512.png` oder `app-icon-1024x1024.png`
+
+### Website-Header
+- `web/logo-horizontal-original.png`
+- alternativ `web/logo-horizontal-512px.png` oder `web/logo-horizontal-1024px.png`
+
+### Social Media / Link-Vorschau
+- `social/og-image-1200x630.png`
+- kleinere Dateigröße: `social/og-image-1200x630.jpg`
+
+## Beispiel für HTML
+
+```html
+<link rel="icon" href="/favicon/favicon.ico" sizes="any">
+<link rel="icon" type="image/png" sizes="32x32" href="/favicon/favicon-32x32.png">
+<link rel="apple-touch-icon" href="/mobile/apple-touch-icon-180x180.png">
+<link rel="manifest" href="/pwa/site.webmanifest">
+
+<meta name="theme-color" content="#248A3D">
+<meta property="og:image" content="/social/og-image-1200x630.png">
+```
+
+Hinweis: Das Ausgangslogo ist ein Rasterbild. Für sehr große Druckprodukte wäre später eine echte SVG-/Vektorversion sinnvoll.

images/foodsharing-manager-logo-assets/pwa/site.webmanifest

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+{
+  "name": "Foodsharing Manager",
+  "short_name": "FS Manager",
+  "icons": [
+    {
+      "src": "/pwa/android-chrome-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/pwa/android-chrome-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/pwa/maskable-icon-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ],
+  "theme_color": "#248A3D",
+  "background_color": "#F7FBF5",
+  "display": "standalone"
+}