fix: improve chat scroll behavior and dark mode readability

- Smart auto-scroll: only follow SSE stream when user is near bottom (200px threshold), scroll once on send/switch session
- Brighten dark mode text colors (primary #e0→#f0, secondary #a0→#c0, muted #66→#88)
- Fix tool-call panel height to match thinking video (120px→213px)
- Fix tool-call item background invisible in dark mode
- Fix gateway start button using hardcoded dark color

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

packages/client/src/styles/variables.scss

@@ -75,9 +75,9 @@
   --accent-muted: #888888;
 
   // Text
-  --text-primary: #e0e0e0;
-  --text-secondary: #a0a0a0;
-  --text-muted: #666666;
+  --text-primary: #f0f0f0;
+  --text-secondary: #c0c0c0;
+  --text-muted: #888888;
 
   // Status
   --success: #66bb6a;
@@ -98,10 +98,10 @@
   --accent-info: #6ba3d6;
 
   // RGB components
-  --accent-primary-rgb: 224, 224, 224;
+  --accent-primary-rgb: 240, 240, 240;
   --accent-hover-rgb: 245, 245, 245;
-  --text-primary-rgb: 224, 224, 224;
-  --text-muted-rgb: 102, 102, 102;
+  --text-primary-rgb: 240, 240, 240;
+  --text-muted-rgb: 136, 136, 136;
   --success-rgb: 102, 187, 106;
   --error-rgb: 239, 83, 80;
   --warning-rgb: 255, 183, 77;