Add comprehensive favicon and PWA support

- Created SVG favicons in multiple sizes (16x16, 32x32, 192x192, 512x512)
- Added web app manifest for PWA functionality
- Implemented Apple Touch Icon support for iOS devices
- Added theme color and mobile web app meta tags
- Used gear/cogs design matching UnitForge branding with blue gradient
- Supports modern browsers with SVG and fallback compatibility
- Enables 'Add to Home Screen' functionality on mobile devices

All favicon files use the same blue gradient (#0d6efd to #0b5ed7)
as the main UI theme for consistent branding.

frontend/static/img/site.webmanifest

@@ -0,0 +1,62 @@
+{
+  "name": "UnitForge",
+  "short_name": "UnitForge",
+  "description": "Systemd Unit File Creator - Create, validate, and manage systemd unit files with ease",
+  "start_url": "/",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#0d6efd",
+  "orientation": "portrait-primary",
+  "categories": ["productivity", "developer", "utilities"],
+  "icons": [
+    {
+      "src": "/static/img/favicon-16x16.svg",
+      "sizes": "16x16",
+      "type": "image/svg+xml"
+    },
+    {
+      "src": "/static/img/favicon.svg",
+      "sizes": "32x32",
+      "type": "image/svg+xml"
+    },
+    {
+      "src": "/static/img/icon-192x192.svg",
+      "sizes": "192x192",
+      "type": "image/svg+xml"
+    },
+    {
+      "src": "/static/img/icon-512x512.svg",
+      "sizes": "512x512",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ],
+  "shortcuts": [
+    {
+      "name": "Visual Editor",
+      "short_name": "Editor",
+      "description": "Create unit files with the visual form editor",
+      "url": "/editor",
+      "icons": [
+        {
+          "src": "/static/img/icon-192x192.svg",
+          "sizes": "96x96",
+          "type": "image/svg+xml"
+        }
+      ]
+    },
+    {
+      "name": "Templates",
+      "short_name": "Templates",
+      "description": "Browse pre-built unit file templates",
+      "url": "/templates",
+      "icons": [
+        {
+          "src": "/static/img/icon-192x192.svg",
+          "sizes": "96x96",
+          "type": "image/svg+xml"
+        }
+      ]
+    }
+  ]
+}