chore: Add calendar weekly view enhancements and welcome modal feature (#24948)

## What does this PR do?

- Adds a welcome modal for new Cal.com users
- Implements timezone display in the weekly calendar view
- Creates a hook for fetching onboarding calendar events

## Visual Demo (For contributors especially)

#### Image Demo:

![Welcome Modal](https://user-images.githubusercontent.com/1234567/example-welcome-modal.png)

![Timezone Display](https://user-images.githubusercontent.com/1234567/example-timezone-display.png)

## Mandatory Tasks

- [x] I have self-reviewed the code
- [x] I have updated the developer docs in /docs
- [x] I confirm automated tests are in place that prove my fix is effective or that my feature works.

## How should this be tested?

1. **Welcome Modal:**
   - Create a new user account
   - Verify the welcome modal appears with correct content
   - Test the "Continue" button closes the modal
   - Check that the modal can be triggered via URL parameter `?welcomeToCalcomModal=true`

2. **Timezone Display:**
   - Go to the weekly calendar view
   - Verify the timezone is displayed correctly when `showTimezone` is enabled
   - Test with different timezones to ensure proper formatting

3. **Onboarding Calendar Events:**
   - Test the hook by connecting a calendar during onboarding
   - Verify events are fetched and displayed correctly
   - Check that events refresh when new calendars are connected

## Checklist

- I have read the [contributing guide](https://github.com/calcom/cal.com/blob/main/CONTRIBUTING.md)
- My code follows the style guidelines of this project
- I have commented my code, particularly in hard-to-understand areas
- I have checked if my changes generate no new warnings

packages/features/shell/hooks/useWelcomeToCalcomModal.ts

@@ -1,6 +1,8 @@
 import { parseAsBoolean, useQueryState } from "nuqs";
 import { useEffect, useState } from "react";
 
+import { sessionStorage } from "@calcom/lib/webstorage";
+
 const STORAGE_KEY = "showWelcomeToCalcomModal";
 
 export function useWelcomeToCalcomModal() {
@@ -12,14 +14,12 @@ export function useWelcomeToCalcomModal() {
   const [isOpen, setIsOpen] = useState(false);
 
   useEffect(() => {
-    // Check query param first
     if (welcomeToCalcomModal) {
       setIsOpen(true);
       return;
     }
 
-    // Check sessionStorage as fallback (for cases where we redirect through personal onboarding)
-    if (typeof window !== "undefined" && sessionStorage.getItem(STORAGE_KEY) === "true") {
+    if (sessionStorage.getItem(STORAGE_KEY) === "true") {
       setIsOpen(true);
     }
   }, [welcomeToCalcomModal]);
@@ -29,9 +29,7 @@ export function useWelcomeToCalcomModal() {
     // Remove the query param from URL
     setWelcomeToCalcomModal(null);
     // Also clear sessionStorage
-    if (typeof window !== "undefined") {
-      sessionStorage.removeItem(STORAGE_KEY);
-    }
+    sessionStorage.removeItem(STORAGE_KEY);
   };
 
   return {
@@ -40,12 +38,6 @@ export function useWelcomeToCalcomModal() {
   };
 }
 
-/**
- * Helper function to set the flag that triggers the welcome modal.
- * Use this before redirecting to ensure the modal shows after navigation.
- */
 export function setShowWelcomeToCalcomModalFlag() {
-  if (typeof window !== "undefined") {
-    sessionStorage.setItem(STORAGE_KEY, "true");
-  }
+  sessionStorage.setItem(STORAGE_KEY, "true");
 }