Task #166: Match menu hover color to Tx OS brand

Changed the shadcn `--accent` token in tx-os from violet-500 (the
shadcn default leftover) to slate-100, matching `--sidebar-accent`
so dropdown / select / context-menu / calendar / popover hover and
focus states now share one cohesive soft-slate look with the
sidebar instead of clashing with the navy header and primary blue.

Files:
- artifacts/tx-os/src/index.css
  - --accent: 262 83% 66% → 210 40% 96%  (slate-100)
  - --accent-foreground: 0 0% 100% → 222 47% 11%  (slate-900)

Cascade: every shadcn primitive that consumes bg-accent /
focus:bg-accent / data-[state=open]:bg-accent picks this up
automatically — no per-component edits needed (verified
DropdownMenu, ContextMenu, NavigationMenu, Menubar, Command,
Calendar, Select, Popover, plus the executive-meetings MergeMenu).

Out of scope (intentionally left): decorative login-page violet
gradients, user-pickable violet color options in the cell/row
color pickers, the violet Bell icon on the notifications page,
and the animated login art.

Pre-existing TS errors in use-notifications-socket.ts and
admin.tsx (missing exports from @workspace/api-client-react) are
codegen drift from the recently merged tasks #109/#110 and are
unrelated to this CSS-only change.

artifacts/tx-os/src/index.css

@@ -74,8 +74,8 @@
   --secondary-foreground: 0 0% 100%;
   --muted: 210 40% 96%;
   --muted-foreground: 215 16% 47%;    /* slate-500 */
-  --accent: 262 83% 66%;              /* violet-500 */
-  --accent-foreground: 0 0% 100%;
+  --accent: 210 40% 96%;              /* slate-100 — matches sidebar hover */
+  --accent-foreground: 222 47% 11%;   /* slate-900 */
   --destructive: 0 84% 60%;
   --destructive-foreground: 0 0% 100%;
   --border: 214 32% 91%;              /* #E2E8F0 */