Onboarding refactor (#39724)

<img width="1648" height="976" alt="Screenshot 2025-10-07 at 6 57 20 PM"
src="https://github.com/user-attachments/assets/ae7289c0-8820-4fdf-ae28-84fb6bd64942"
/>

Fixes #39347

Release Notes:

- Improved onboarding UI by collapsing it to a single page

---------

Co-authored-by: dino <dinojoaocosta@gmail.com>
Co-authored-by: Lukas Wirth <me@lukaswirth.dev>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>

assets/keymaps/default-linux.json

@@ -1229,9 +1229,6 @@
     "context": "Onboarding",
     "use_key_equivalents": true,
     "bindings": {
-      "ctrl-1": "onboarding::ActivateBasicsPage",
-      "ctrl-2": "onboarding::ActivateEditingPage",
-      "ctrl-3": "onboarding::ActivateAISetupPage",
       "ctrl-enter": "onboarding::Finish",
       "alt-shift-l": "onboarding::SignIn",
       "alt-shift-a": "onboarding::OpenAccount"