S1: frontend infra — AsyncState/EmptyState + useNoteEditor, adopted in BoardView

M9 section S1, commit 4 (frontend). Introduce the shared UI primitives the 7 data
views + 5 editor hosts were hand-rolling:

- components/AsyncState.vue: the loading / error(+Retry) wrapper (emits `retry`).
- components/EmptyState.vue: the centered title/subtitle "nothing here" block.
- composables/useNoteEditor.ts: the editing/open/close/navigate glue every editor
  host duplicated, as one controller (onClose hook + local-list resolution for
  [[wiki-link]] navigation).

BoardView adopts all three: its three hand-rolled loading/error/empty blocks
collapse into <AsyncState> + <EmptyState>, and its editor glue into useNoteEditor.
The other views + editor hosts adopt these in the Organize (S3) and Auth (S5)
sections. Behavior-preserving.

Frontend has no local typecheck (rule 10); CI's vue-tsc is the gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/AsyncState.vue

@@ -0,0 +1,27 @@
+<script setup lang="ts">
+// The loading / error(+retry) wrapper every data view hand-rolled. While `loading`
+// shows a spinner line; on `error` shows the message + a Retry button (emits
+// `retry`); otherwise renders the default slot (the caller decides empty vs content).
+defineProps<{ loading: boolean; error?: string; errorTitle?: string }>();
+defineEmits<{ (e: "retry"): void }>();
+</script>
+
+<template>
+  <div v-if="loading" class="py-24 text-center text-sm text-neutral-400">Loading…</div>
+
+  <div v-else-if="error" class="py-24 text-center">
+    <h2 class="text-lg font-semibold text-neutral-700 dark:text-neutral-200">
+      {{ errorTitle ?? "Something went wrong" }}
+    </h2>
+    <p class="mt-1 text-sm text-neutral-400">{{ error }}</p>
+    <button
+      type="button"
+      class="mt-3 rounded-md border border-neutral-300 px-3 py-1.5 text-sm hover:bg-neutral-100 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand dark:border-neutral-700 dark:hover:bg-neutral-800"
+      @click="$emit('retry')"
+    >
+      Retry
+    </button>
+  </div>
+
+  <slot v-else />
+</template>

frontend/src/components/EmptyState.vue

@@ -0,0 +1,13 @@
+<script setup lang="ts">
+// The centered "nothing here" block every data view renders. Title + optional
+// subtitle; an optional default slot for a call-to-action beneath.
+defineProps<{ title: string; subtitle?: string }>();
+</script>
+
+<template>
+  <div class="py-24 text-center">
+    <h2 class="text-lg font-semibold text-neutral-700 dark:text-neutral-200">{{ title }}</h2>
+    <p v-if="subtitle" class="mt-1 text-sm text-neutral-400">{{ subtitle }}</p>
+    <div v-if="$slots.default" class="mt-3"><slot /></div>
+  </div>
+</template>