Improve binder underlines and hover styling (#153)

## Description
Match clickable binder underline colors to their text and show a subtle
tint of the selected background when hovering over a binder. Keep
source-highlight underlines dotted on hover.

<img width="408" height="161" alt="image"
src="https://github.com/user-attachments/assets/17e21380-f29d-45df-b7a2-2ec574149e36"
/>

Validated with a production extension build and independent review of
the split.

🤖 Generated with [Codex](https://openai.com/codex/)

---------

Co-authored-by: Codex <noreply@openai.com>

Author: rcosta358

client/src/webview/styles.ts

@@ -826,7 +826,12 @@ export function getStyles(): string {
         }
         .highlight-var-btn.clickable:hover {
             font-weight: inherit;
-            text-decoration-style: solid;
+        }
+        .vc-binder.highlight-var-btn:not(.selected):hover {
+            background-color: color-mix(in srgb, var(--vscode-button-background) 35%, transparent);
+        }
+        .vc-binder.highlight-var-btn.clickable {
+            text-decoration-color: currentColor;
         }
         .vc-binder.highlight-var-btn.selected {
             color: var(--lj-token-identifier);