feat(stores): add Pinia domain stores and update docs

Implement concrete Pinia stores for app UI and domain data instead of
placeholder re-exports, including seeded student records and snackbar state.

Refresh README guidance for components, plugins, and services to document the
current project structure, data flow, and usage conventions.feat(stores): add Pinia domain stores and update docs

Implement concrete Pinia stores for app UI and domain data instead of
placeholder re-exports, including seeded student records and snackbar state.

Refresh README guidance for components, plugins, and services to document the
current project structure, data flow, and usage conventions.

src/stores/snackbar.ts

@@ -1 +1,52 @@
-export * from './stores/snackbar'
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+
+type SnackbarColor = string
+type SnackbarVariant = 'flat' | 'text' | 'elevated' | 'tonal' | 'outlined' | 'plain'
+
+type SnackbarLocation = string
+
+interface ShowOptions {
+  message: string
+  color?: SnackbarColor
+  timeout?: number
+  location?: SnackbarLocation
+  variant?: SnackbarVariant
+}
+
+export const useSnackbarStore = defineStore('snackbar', () => {
+  const visible = ref(false)
+  const message = ref('')
+  const color = ref<SnackbarColor>('success')
+  const timeout = ref(2000)
+  const location = ref<SnackbarLocation>('top right')
+  const variant = ref<SnackbarVariant>('flat')
+
+  const show = (options: ShowOptions) => {
+    message.value = options.message
+    color.value = options.color ?? 'success'
+    timeout.value = options.timeout ?? 2000
+    location.value = options.location ?? 'top right'
+    variant.value = options.variant ?? 'flat'
+
+    visible.value = false
+    requestAnimationFrame(() => {
+      visible.value = true
+    })
+  }
+
+  const hide = () => {
+    visible.value = false
+  }
+
+  return {
+    visible,
+    message,
+    color,
+    timeout,
+    location,
+    variant,
+    show,
+    hide,
+  }
+})