feat(web): switch the accent colour to Claude's terracotta orange

Both themes use #d97757 for the accent. Buttons carry the dark ink colour instead of white, since white on this orange falls short of readable contrast. The favicon follows.

web/src/app.css

@@ -10,9 +10,11 @@
 	--border: #d8d5cb;
 	--text: #1d1c19;
 	--text-muted: #6b675d;
-	--accent: #2f6f4f;
-	--accent-text: #ffffff;
-	--accent-soft: #dcece3;
+	/* Claude's terracotta orange. Light enough that white text on it fails
+	   contrast, so buttons carry the ink colour instead. */
+	--accent: #d97757;
+	--accent-text: #1d1c19;
+	--accent-soft: #f7e7df;
 	--danger: #b3261e;
 	--danger-soft: #fbe4e2;
 	--warn: #b06d0a;
@@ -41,9 +43,9 @@
 	--border: #333840;
 	--text: #e9e9e6;
 	--text-muted: #9aa0a8;
-	--accent: #5fb98a;
-	--accent-text: #10241a;
-	--accent-soft: #1f3a2c;
+	--accent: #d97757;
+	--accent-text: #1d1c19;
+	--accent-soft: #3b2a23;
 	--danger: #f2867d;
 	--danger-soft: #3a201e;
 	--warn: #e0a94a;

web/static/favicon.svg

@@ -1,5 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
-  <rect width="64" height="64" rx="14" fill="#2f6f4f"/>
+  <rect width="64" height="64" rx="14" fill="#d97757"/>
   <text x="32" y="44" font-family="system-ui, sans-serif" font-size="34" font-weight="700"
-        text-anchor="middle" fill="#ffffff">NT</text>
+        text-anchor="middle" fill="#1d1c19">NT</text>
 </svg>