fix(ui): readable selection colours, unified agent control sizing and Home width (#2064)

Closes #1985

The SQL console and GraphQL editors draw transparent text over their highlighting, so a selection showed only an empty highlight; they now get selection colours from the tokens, so selected text stays readable in light and dark themes. Agent action buttons share one size, and the Home composer uses the same width token as the conversation composer. Verified on macOS in light and dark themes.

Co-authored-by: panda-student <[email protected]>

src/features/graphql/GraphQL.module.css

@@ -196,7 +196,7 @@
 }
 .variables::selection {
   background: var(--bg-editor-selection);
-  color: transparent;
+  color: var(--fg);
 }
 
 /*