assistant2: Adjust edit files design (#27762)

This PR includes design tweaks to elements involved on the "edit files"
flow: the bar that appears above the message editor, buttons on the
multibuffer hunks, adding keybindings to the "Review Changes" button,
etc.

<img
src="https://github.com/user-attachments/assets/4bff883a-c5c4-443e-8bf5-d98f535c83ce"
width="750" />

Release Notes:

- N/A

assets/keymaps/default-linux.json

@@ -651,7 +651,8 @@
     "context": "MessageEditor > Editor",
     "bindings": {
       "enter": "assistant2::Chat",
-      "ctrl-i": "assistant2::ToggleProfileSelector"
+      "ctrl-i": "assistant2::ToggleProfileSelector",
+      "shift-ctrl-r": "assistant2::OpenAssistantDiff"
     }
   },
   {