DEV: implement composer peek mode directly in theme (#141)

This implements the [composer
peek](https://github.com/discourse/composer-peek) component directly
into the theme, and makes some minor adjustments to match theme styles:


![image](https://github.com/user-attachments/assets/e4272a11-6570-4392-945f-c509cf4debd6)

common/common.scss

@@ -3,6 +3,7 @@
 @import "chat";
 @import "color-choice";
 @import "composer";
+@import "composer-peek-mode";
 @import "header";
 @import "hiddenstuff";
 @import "login";