Add design tweaks to the AI configuration panel (#15894)

This PR polishes elements around setting up LLM providers on the
Assistant panel, including:

- [x] Adding banners for promoting Zed AI and to deal with the "No
provider set up" scenario
- [x] Tweaking the error popover whenever there's no API key added
- [ ] Making configuration panel scrollable

--- 

Release Notes:

- N/A

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet Bo Fenner <53836821+bennetbo@users.noreply.github.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

crates/language_model/src/provider/google.rs

@@ -14,7 +14,7 @@ use settings::{Settings, SettingsStore};
 use std::{future, sync::Arc, time::Duration};
 use strum::IntoEnumIterator;
 use theme::ThemeSettings;
-use ui::{prelude::*, Indicator};
+use ui::{prelude::*, Icon, IconName};
 use util::ResultExt;
 
 use crate::{
@@ -454,9 +454,9 @@ impl Render for ConfigurationView {
                 .justify_between()
                 .child(
                     h_flex()
-                        .gap_2()
-                        .child(Indicator::dot().color(Color::Success))
-                        .child(Label::new("API key configured").size(LabelSize::Small)),
+                        .gap_1()
+                        .child(Icon::new(IconName::Check).color(Color::Success))
+                        .child(Label::new("API key configured.")),
                 )
                 .child(
                     Button::new("reset-key", "Reset key")