fix(theme): improve forest theme readability and contrast

- Lighten --color-muted from #6b9a77 to #a3c9a8 — prose body text and
  secondary text were blending into the near-black green background
- Surface colors lifted slightly for card depth distinction
- Border color made more visible (#1e3a24 → #2c4e34)
- brand-dim updated to green-500 for slightly brighter hover states

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/app.css

@@ -100,13 +100,13 @@
 /* ── Forest theme — dark green ────────────────────────────────────────── */
 [data-theme="forest"] {
 	--color-brand:     #4ade80;   /* green-400 */
-	--color-brand-dim: #16a34a;   /* green-600 */
+	--color-brand-dim: #22c55e;   /* green-500 — brighter than green-600 for hover */
 	--color-surface:   #0a130d;   /* custom near-black green */
-	--color-surface-2: #111c14;   /* custom dark green */
-	--color-surface-3: #1a2e1e;   /* custom mid green */
-	--color-muted:     #6b9a77;   /* custom muted green */
-	--color-text:      #e8f5e9;   /* custom light green-tinted white */
-	--color-border:    #1e3a24;   /* custom green border */
+	--color-surface-2: #14201a;   /* custom dark green — lifted slightly for card depth */
+	--color-surface-3: #1f3326;   /* custom mid green — more contrast from surface-2 */
+	--color-muted:     #a3c9a8;   /* lightened: readable body text, still green-tinted */
+	--color-text:      #eaf5eb;   /* near-white with green tint */
+	--color-border:    #2c4e34;   /* more visible green border */
 	--color-danger:    #f87171;   /* red-400 */
 	--color-success:   #4ade80;   /* green-400 */
 }