chore(agent): move screen.html inline styles into agent.css; add --blue/--yellow/--subtext0 to palette

frontend/packages/shared/src/base.css

@@ -11,11 +11,14 @@
   --purple: #cba6f7;    /* mauve */
   --purple-dim: #45475a;/* surface1 */
   --cyan: #89dceb;      /* sky */
+  --blue: #89b4fa;      /* blue */
   --pink: #f5c2e7;      /* pink */
   --amber: #fab387;     /* peach */
+  --yellow: #f9e2af;    /* yellow */
   --green: #a6e3a1;     /* green */
   --red: #f38ba8;       /* red */
   --border: #313244;    /* surface0 */
+  --subtext0: #a6adc8;  /* subtext0 — secondary/toolbar text, dimmer than --fg but lighter than --muted */
 }
 body {
   background: var(--bg);