Notification provider: pluggable notify() instead of hardcoded sonner toasts in every plugin

[olliethedev]

Context

Part of the Refine-inspired core simplification effort (#127 ✅, #128, #129, #130 ✅, #132, #133). Refine's mutations report success/failure through a pluggable notificationProvider instead of binding a toast library inside the framework.

Scope note: this issue covers the core provider only (phase 1). The sweep replacing 50+ direct sonner call sites across ~15 files in 9 plugins is tracked in #136 (phase 2).

Problem

Plugin components call toast.success(...) / toast.error(...) directly:

• Consumers cannot route plugin notifications through their own notification system (or suppress/translate them)
• Sonner is effectively a hard dependency of every plugin UI
• Success/error messaging is scattered through components as try/catch + toast instead of living with mutation definitions

Proposal

A notify provider on StackProvider, with sonner as the default so behavior is unchanged out of the box:

<StackProvider
  notify={{
    success: (msg, opts) => myToast.success(msg),
    error: (msg, opts) => myToast.error(msg),
    // optional: info, warning
  }}
  ...
>

• Core exposes useNotify() from @btst/stack/context
• Integration point for Internal resource-hook factory: deduplicate query-keys/hooks boilerplate across plugins #129: createResource mutations / useForm accept declarative successMessage / errorMessage routed through notify
• Message strings stay key-friendly so they can route through the i18n provider (i18n provider: translatable plugin UI strings with English defaults #132)

Scope (core only)

• notify prop on StackProvider + useNotify() hook + sonner default implementation
• Unit tests: custom provider receives notifications; default falls back to sonner
• Docs: StackProvider reference + short "custom notifications" guide

Acceptance criteria

• Apps without a notify override see identical toast behavior to today
• A consumer-provided notify receives all notifications emitted through useNotify()
• Internal resource-hook factory: deduplicate query-keys/hooks boilerplate across plugins #129's useForm/mutations can emit through the provider

Plugin sweep (removing direct sonner imports) → #136.