feat: app-wide toast notifications

Adds a client-side toast store (info/success/error, auto-dismiss with a
sticky option) and a Toaster component mounted in the root layout, wired to
the reserved --z-toast token. Gives mutations a consistent way to surface
success/failure instead of inline-only text or silent catches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.114.2",
+  "version": "0.115.0",
   "private": true,
   "type": "module",
   "scripts": {