Swap the start building and login buttons (#39576)

New onboarding screen:

<img width="1027" height="700" alt="Screenshot 2025-10-05 at 10 38
57 PM"
src="https://github.com/user-attachments/assets/5dc49e53-68e7-4559-8ce0-1bada629781d"
/>


This PR also adds a new telemetry event: `Welcome Start Building
Clicked`

Release Notes:

- N/A

crates/paths/src/paths.rs

@@ -65,7 +65,7 @@ pub fn set_custom_data_dir(dir: &str) -> &'static PathBuf {
     }
     CUSTOM_DATA_DIR.get_or_init(|| {
         let mut path = PathBuf::from(dir);
-        if path.is_relative() {
+        if path.is_relative() && path.exists() {
             let abs_path = path
                 .canonicalize()
                 .expect("failed to canonicalize custom data directory's path to an absolute path");