Improve PWA install experience for Khoj on Desktop, Mobile

- Resolve PWA issues thrown by Chrome/Edge
  - Add screenshot samples showcasing remember, browse and draw features
    - This can provide a richer app store like experience when
      installing Khoj PWA on Mobile or Desktop
    - Add wide and narrow screenshots to show Mobile vs Desktop UX
  - Add higher resolution favicon for PWA
- Use single web manifest instead of separate ones for Chat, Search
- Update manifest description with more details about Khoj features

src/interface/desktop/about.html

@@ -5,7 +5,7 @@
         <title>Khoj - About</title>
 
         <link rel="icon" type="image/png" sizes="128x128" href="./assets/icons/favicon-128x128.png">
-        <link rel="manifest" href="/static/khoj_chat.webmanifest">
+        <link rel="manifest" href="/static/khoj.webmanifest">
         <link rel="stylesheet" href="./assets/khoj.css">
      </head>
     <script type="text/javascript" src="./utils.js"></script>

src/interface/desktop/chat.html

@@ -5,7 +5,7 @@
         <title>Khoj - Chat</title>
 
         <link rel="icon" type="image/png" sizes="128x128" href="./assets/icons/favicon-128x128.png">
-        <link rel="manifest" href="/static/khoj_chat.webmanifest">
+        <link rel="manifest" href="/static/khoj.webmanifest">
         <link rel="stylesheet" href="./assets/khoj.css">
     </head>
     <script type="text/javascript" src="./assets/markdown-it.min.js"></script>

src/khoj/interface/web/chat.html

@@ -5,7 +5,7 @@
         <title>Khoj - Chat</title>
 
         <link rel="icon" type="image/png" sizes="128x128" href="/static/assets/icons/favicon-128x128.png">
-        <link rel="manifest" href="/static/khoj_chat.webmanifest">
+        <link rel="manifest" href="/static/khoj.webmanifest">
         <link rel="stylesheet" href="/static/assets/khoj.css">
     </head>
     <script type="text/javascript" src="/static/assets/utils.js"></script>

src/khoj/interface/web/khoj.webmanifest

@@ -1,16 +1,51 @@
 {
     "name": "Khoj",
     "short_name": "Khoj",
-    "description": "An AI search assistant for your digital brain",
+    "display": "standalone",
+    "start_url": "/",
+    "description": "The open, personal AI for your digital brain. You can ask Khoj to draft a message, paint your imagination, find information on the internet and even answer questions from your documents.",
+    "theme_color": "#ffffff",
+    "background_color": "#ffffff",
     "icons": [
         {
             "src": "/static/assets/icons/favicon-128x128.png",
             "sizes": "128x128",
             "type": "image/png"
+        },
+        {
+            "src": "/static/assets/icons/favicon-256x256.png",
+            "sizes": "256x256",
+            "type": "image/png"
         }
     ],
-    "theme_color": "#ffffff",
-    "background_color": "#ffffff",
-    "display": "standalone",
-    "start_url": "/"
+    "screenshots": [
+        {
+         "src": "/static/assets/samples/phone-remember-plan-sample.png",
+          "sizes": "419x900",
+          "type": "image/png",
+          "form_factor": "narrow",
+          "label": "Remember and Plan"
+        },
+        {
+         "src": "/static/assets/samples/phone-browse-draw-sample.png",
+          "sizes": "419x900",
+          "type": "image/png",
+          "form_factor": "narrow",
+          "label": "Browse and Draw"
+        },
+        {
+         "src": "/static/assets/samples/desktop-remember-plan-sample.png",
+          "sizes": "1260x742",
+          "type": "image/png",
+          "form_factor": "wide",
+          "label": "Remember and Plan"
+        },
+        {
+         "src": "/static/assets/samples/desktop-browse-draw-sample.png",
+          "sizes": "1260x742",
+          "type": "image/png",
+          "form_factor": "wide",
+          "label": "Browse and Draw"
+        }
+    ]
 }

src/khoj/interface/web/khoj_chat.webmanifest

@@ -1,16 +0,0 @@
-{
-    "name": "Khoj Chat",
-    "short_name": "Khoj Chat",
-    "description": "An AI personal assistant for your digital brain",
-    "icons": [
-        {
-            "src": "/static/assets/icons/favicon-128x128.png",
-            "sizes": "128x128",
-            "type": "image/png"
-        }
-    ],
-    "theme_color": "#ffffff",
-    "background_color": "#ffffff",
-    "display": "standalone",
-    "start_url": "/chat"
-}