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/templates/index.html

@@ -4,6 +4,15 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>UnitForge - Systemd Unit File Creator</title>
+    <link rel="icon" type="image/svg+xml" href="/static/img/favicon.svg">
+    <link rel="icon" type="image/svg+xml" sizes="16x16" href="/static/img/favicon-16x16.svg">
+    <link rel="alternate icon" href="/static/img/favicon.svg">
+    <link rel="apple-touch-icon" href="/static/img/icon-192x192.svg">
+    <link rel="manifest" href="/static/img/site.webmanifest">
+    <meta name="theme-color" content="#0d6efd">
+    <meta name="apple-mobile-web-app-capable" content="yes">
+    <meta name="apple-mobile-web-app-status-bar-style" content="default">
+    <meta name="apple-mobile-web-app-title" content="UnitForge">
     <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/css/bootstrap.min.css" rel="stylesheet">
     <link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.4.0/css/all.min.css" rel="stylesheet">
     <link href="/static/css/style.css" rel="stylesheet">