Add refinements to the AI onboarding flow (#33738)

This includes making sure that both the agent panel and Zed's edit
prediction have a consistent narrative when it comes to onboarding users
into the AI features, considering the possible different plans and
conditions (such as being signed in/out, account age, etc.)

Release Notes:

- N/A

---------

Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/client/src/zed_urls.rs

@@ -17,3 +17,8 @@ fn server_url(cx: &App) -> &str {
 pub fn account_url(cx: &App) -> String {
     format!("{server_url}/account", server_url = server_url(cx))
 }
+
+/// Returns the URL to the upgrade page on zed.dev.
+pub fn upgrade_to_zed_pro_url(cx: &App) -> String {
+    format!("{server_url}/account/upgrade", server_url = server_url(cx))
+}