[web] Update Share modal paywall CTAs to point to the plans interstitial (#34410)

* [web] Send paywall upgrade CTAs to the plans interstitial

* fix: encode upgradePlan campaign source with URLSearchParams

Co-authored-by: lzear <8427791+lzear@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
GitOrigin-RevId: ddde4d77d72e649f5ded5839a2a2155e5f679f53

services/web/frontend/js/main/account-upgrade.ts

@@ -39,6 +39,10 @@ export function upgradePlan(source: string) {
   }
 
   if (openedWindow) {
-    openedWindow.location = '/user/subscription'
+    const searchParams = new URLSearchParams({
+      itm_campaign: source,
+    })
+
+    openedWindow.location = `/user/subscription/choose-your-plan?${searchParams.toString()}`
   }
 }