Fix conversation selector popover menu offset (#22796)

Before, the conversation popover menu covered up what you were typing
because it wasn't offset properly.

Now it's offset properly, using the UI font size so the amount of offset
scales with the font size:

<img width="435" alt="Screenshot 2025-01-07 at 4 34 27 PM"
src="https://github.com/user-attachments/assets/55e40910-8cd4-4548-b4fb-521eb2845775"
/>
<img width="454" alt="Screenshot 2025-01-07 at 4 33 58 PM"
src="https://github.com/user-attachments/assets/30350489-09f1-4cb8-9f95-ed4ee87bc110"
/>
<img width="488" alt="Screenshot 2025-01-07 at 4 34 18 PM"
src="https://github.com/user-attachments/assets/de60d990-2bd9-418d-a616-56beb3e4aa8a"
/>

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/assistant2/src/message_editor.rs

@@ -10,7 +10,7 @@ use language_model::{LanguageModelRegistry, LanguageModelRequestTool};
 use language_model_selector::LanguageModelSelector;
 use rope::Point;
 use settings::Settings;
-use theme::ThemeSettings;
+use theme::{get_ui_font_size, ThemeSettings};
 use ui::{
     prelude::*, ButtonLike, ElevationIndex, KeyBinding, PopoverMenu, PopoverMenuHandle,
     SwitchWithLabel,
@@ -276,7 +276,7 @@ impl Render for MessageEditor {
                             .anchor(gpui::Corner::BottomLeft)
                             .offset(gpui::Point {
                                 x: px(0.0),
-                                y: px(-16.0),
+                                y: (-get_ui_font_size(cx) * 2) - px(4.0),
                             })
                             .with_handle(self.inline_context_picker_menu_handle.clone()),
                     )