Improve Khoj server status check in Khoj Obsidian client

- Update server connection status on every edit of khoj url, api key in
  settings instead of only on plugin load

  The error message was stale if connection fixed after changes in
  Khoj plugin settings to URL or API key, like on plugin install

- Show better welcome message on first plugin install.
  Include API key setup instruction

- Show logged in user email on Khoj settings page

src/interface/obsidian/src/utils.ts

@@ -1,4 +1,4 @@
-import { FileSystemAdapter, Notice, Vault, Modal, TFile } from 'obsidian';
+import { FileSystemAdapter, Notice, Vault, Modal, TFile, request } from 'obsidian';
 import { KhojSetting } from 'src/settings'
 
 export function getVaultAbsolutePath(vault: Vault): string {
@@ -123,3 +123,50 @@ export async function createNoteAndCloseModal(query: string, modal: Modal, opt?:
     }
     modal.close();
 }
+
+export async function canConnectToBackend(
+    khojUrl: string,
+    khojApiKey: string,
+    showNotice: boolean = false
+): Promise<{ connectedToBackend: boolean; statusMessage: string, userEmail: string }> {
+    let connectedToBackend = false;
+    let userEmail: string = '';
+
+    if (!!khojUrl) {
+        let headers  = !!khojApiKey ? { "Authorization": `Bearer ${khojApiKey}` } : undefined;
+        await request({ url: `${khojUrl}/api/health`, method: "GET", headers: headers })
+        .then(response => {
+            connectedToBackend = true;
+            userEmail = JSON.parse(response)?.email;
+        })
+        .catch(error => {
+            connectedToBackend = false;
+            console.log(`Khoj connection error:\n\n${error}`);
+        });
+    }
+
+    let statusMessage: string = getBackendStatusMessage(connectedToBackend, userEmail, khojUrl, khojApiKey);
+    if (showNotice) new Notice(statusMessage);
+    return { connectedToBackend, statusMessage, userEmail };
+}
+
+export function getBackendStatusMessage(
+    connectedToServer: boolean,
+    userEmail: string,
+    khojUrl: string,
+    khojApiKey: string
+): string {
+    // Welcome message with default settings. Khoj cloud always expects an API key.
+    if (!!khojApiKey && khojUrl === 'https://app.khoj.dev')
+        return `🌈 Welcome to Khoj! Get your API key from ${khojUrl}/config#clients and set it in the Khoj plugin settings on Obsidian`;
+
+    if (!connectedToServer)
+        return `❗️Could not connect to Khoj at ${khojUrl}. Ensure your can access it`;
+    else if (!userEmail)
+        return `✅ Connected to Khoj. ❗️Get a valid API key from ${khojUrl}/config#clients to log in`;
+    else if (userEmail === 'default@example.com')
+        // Logged in as default user in anonymous mode
+        return `✅ Signed in to Khoj`;
+    else
+        return `✅ Signed in to Khoj as ${userEmail}`;
+}