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>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.114.2"
+version = "0.115.0"
 edition = "2021"
 default-run = "mangalord"