Add deprecation notice for 5.x users (#2904)

Show a modal on app startup informing users that Quiet 5.x is deprecated
due to a security fix in 6.0 and they should upgrade.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: holmesworcester

packages/desktop/src/renderer/containers/widgets/breakingChangesWarning/BreakingChangesWarning.tsx

@@ -15,12 +15,16 @@ const BreakingChangesWarning = () => {
 
   const title = 'Update available'
   const message =
-    'Quiet’s next release makes joining communities faster and more reliable by letting people join when the owner is offline! 🎉 However, these changes are not backwards compatible, so you must re-install Quiet from tryquiet.org and re-create or re-join your community. 😥 This version of Quiet will no longer receive any updates or security fixes, so please re-install soon. We apologize for the inconvenience.'
+    'Quiet 6.0 includes a security fix that is not backwards compatible, so you must re-install Quiet from tryquiet.org and re-create or re-join your community. This version of Quiet (5.x) will no longer receive any updates or security fixes, so please re-install soon. We apologize for the inconvenience.'
 
   const updateAction = useCallback(() => {
     shell.openExternal(`${Site.MAIN_PAGE}#Downloads`)
   }, [])
 
+  useEffect(() => {
+    modal.handleOpen() // Open modal once per app start
+  }, [])
+
   const updateButton = (
     <Button
       variant='contained'
@@ -35,7 +39,7 @@ const BreakingChangesWarning = () => {
       }}
       fullWidth
     >
-      Install Quiet 2.x
+      Install Quiet 6.x
     </Button>
   )