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>
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