docs: point all Slack invite links to the current invite

PR #410 updated only the footer Slack link to the new invite
(zt-411pbtz7r), leaving the badge (line 17) and the inline link
(line 186) pointing at the old, expired invite (zt-3q7kj5gr6).
Readers clicking those hit "link expired" while the footer link
worked, so the same README appeared to have both broken and
working Slack links. Align all three on the current invite.

Co-Authored-By: Dishit Karia <hanmadishit74@gmail.com>

Author: dishit-wednesday

README.md

@@ -14,7 +14,7 @@
 [![App Store](https://img.shields.io/badge/App%20Store-Download-blue?logo=apple)](https://apps.apple.com/us/app/off-grid-local-ai/id6759299882)
 [![Platform](https://img.shields.io/badge/Platform-Android%20%7C%20iOS%20%7C%20macOS-green.svg)](#install)
 [![codecov](https://codecov.io/gh/alichherawalla/off-grid-mobile/graph/badge.svg)](https://codecov.io/gh/alichherawalla/off-grid-mobile)
-[![Slack](https://img.shields.io/badge/Slack-Join%20Community-4A154B?logo=slack)](https://join.slack.com/t/off-grid-mobile/shared_invite/zt-3q7kj5gr6-rVzx5gl5LKPQh4mUE2CCvA)
+[![Slack](https://img.shields.io/badge/Slack-Join%20Community-4A154B?logo=slack)](https://join.slack.com/t/off-grid-mobile/shared_invite/zt-411pbtz7r-lcOK4YCeY40vh_~FUdcvLA)
 [![Pro](https://img.shields.io/badge/Off%20Grid%20Pro-%2450%20lifetime-000000?style=flat)](https://offgridmobileai.co/pay/)
 
 </div>
@@ -183,7 +183,7 @@ This project is tested with BrowserStack.
 
 ## Community
 
-Join the conversation on [**Slack**](https://join.slack.com/t/off-grid-mobile/shared_invite/zt-3q7kj5gr6-rVzx5gl5LKPQh4mUE2CCvA) — ask questions, share feedback, and connect with other Off Grid users and contributors.
+Join the conversation on [**Slack**](https://join.slack.com/t/off-grid-mobile/shared_invite/zt-411pbtz7r-lcOK4YCeY40vh_~FUdcvLA) — ask questions, share feedback, and connect with other Off Grid users and contributors.
 
 ---