Combine Rules and Admin corner links into one upper-right menu

Replace the fixed Rules link (upper-left) and the creator-only Admin
link (upper-right) with a single collapsible ☰ Menu in the upper right,
leaving the event log alone in its corner. CornerMenu always lists the
Rules link; pages can contribute context-specific entries through the
extraMenuLinks store, which Dashboard uses for the creator-only Admin
link. This gives future misc sub-pages a home without eating more
screen corners.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/menu.js

@@ -0,0 +1,6 @@
+import { writable } from 'svelte/store';
+
+// Extra entries for the corner menu, set by pages that have context-specific
+// links (e.g. Dashboard adds the creator-only Admin link). Each entry is
+// { label, href, external?, title? }.
+export const extraMenuLinks = writable([]);