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/views/hermes/GatewaysView.vue

@@ -54,8 +54,7 @@ async function handleToggle(name: string, running: boolean) {
               </NTag>
               <NButton
                 size="small"
-                :type="gw.running ? 'warning' : 'default'"
-                :color="gw.running ? undefined : '#18181b'"
+                :type="gw.running ? 'warning' : 'primary'"
                 round
                 @click="handleToggle(gw.profile, gw.running)"
               >