assistant panel: Tab-less configuration view (#15682)

TODOs for follow-up:
- [ ] When opening panel: nudge user to sign in if they're not signed-in
and have no provider configured (or if they're not signed-in and have
Zed AI configured)
- [ ] Configuration page is not scrollable
- [ ] Design tweaks

Current status:



https://github.com/user-attachments/assets/d26d65ea-43e8-481b-81a3-b3cba01704a8


Release Notes:

- N/A

crates/language_model/src/registry.rs

@@ -166,11 +166,8 @@ impl LanguageModelRegistry {
             .collect()
     }
 
-    pub fn provider(
-        &self,
-        name: &LanguageModelProviderId,
-    ) -> Option<Arc<dyn LanguageModelProvider>> {
-        self.providers.get(name).cloned()
+    pub fn provider(&self, id: &LanguageModelProviderId) -> Option<Arc<dyn LanguageModelProvider>> {
+        self.providers.get(id).cloned()
     }
 
     pub fn select_active_model(