style: mule ASCII placeholder in sidebar, bump muted text contrast

Replace Info icon with braille mule art in the right sidebar empty
state. Lighten text-muted (#a8997d → #c4b599) and text-faint
(#5e5448 → #7a6e5e) for better readability across the app.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/index.css

@@ -10,8 +10,8 @@
     --color-surface-offset: #352c46;
     --color-border: rgba(240, 193, 136, 0.10);
     --color-text: #f0e6d2;
-    --color-text-muted: #a8997d;
-    --color-text-faint: #5e5448;
+    --color-text-muted: #c4b599;
+    --color-text-faint: #7a6e5e;
     --color-primary: #e8965a;
     --color-pick: #8aaa5a;
     --color-reject: #d06464;