fix(settings): prevent project-level global keys from overriding global settings

getSettingsFast() and getSettings() spread project-level settings over
global settings without filtering by scope. A stale project-level
experimentalFeatures: {} in SQLite would override the global
experimentalFeatures: { insights: true, ... }, making all experimental
features invisible in the dashboard.

Now both methods strip global-only keys from project settings before
the merge, matching the behavior of getSettingsByScopeFast().

Also adds max-height and overflow-y to the overflow dropdown menu.

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

packages/dashboard/app/components/Header.css

@@ -702,6 +702,8 @@
   border-radius: var(--radius-md);
   box-shadow: var(--shadow-lg);
   min-width: 140px;
+  max-height: min(400px, 70vh);
+  overflow-y: auto;
   z-index: 200;
   padding: var(--space-xs) 0;
 }