feat(stacked-note): surface note freshness and guard saves on conflict

Adds a Tabler-icon badge in the stacked-note action bar showing whether
the loaded copy still matches GitHub HEAD (verified / outdated / offline
/ checking / unknown / stale-known). The save flow now re-checks before
the PUT and opens a conflict modal when GitHub has moved on, with three
explicit choices: discard local edits and pull, overwrite anyway, or
cancel. Race-condition 409s from the PUT itself are routed through the
same modal.

src/components/NoteConflictModal.vue

@@ -0,0 +1,82 @@
+<script lang="ts" setup>
+import { onMounted, ref, watch } from "vue"
+
+const props = defineProps<{ open: boolean }>()
+
+const emit = defineEmits<{
+  (e: "discard"): void
+  (e: "overwrite"): void
+  (e: "cancel"): void
+  (e: "update:open", value: boolean): void
+}>()
+
+const dialogRef = ref<HTMLDialogElement | null>(null)
+
+const close = () => {
+  if (dialogRef.value?.open) dialogRef.value.close()
+  emit("update:open", false)
+}
+
+const choose = (action: "discard" | "overwrite" | "cancel") => {
+  emit(action)
+  close()
+}
+
+watch(
+  () => props.open,
+  (open) => {
+    const el = dialogRef.value
+    if (!el) return
+    if (open && !el.open) el.showModal()
+    else if (!open && el.open) el.close()
+  }
+)
+
+onMounted(() => {
+  if (props.open) dialogRef.value?.showModal()
+})
+</script>
+
+<template>
+  <dialog
+    ref="dialogRef"
+    class="modal"
+    @close="emit('update:open', false)"
+    @cancel.prevent="choose('cancel')"
+  >
+    <div class="modal-box">
+      <h3 class="text-lg font-bold">GitHub has a newer version of this note</h3>
+      <p class="py-3 text-sm">
+        Someone (or another device) updated this note on GitHub since you
+        started editing. If you save now, their changes will be overwritten.
+      </p>
+
+      <div class="modal-action flex-col gap-2 sm:flex-row sm:justify-end">
+        <button
+          type="button"
+          class="btn btn-ghost"
+          @click="choose('cancel')"
+        >
+          Cancel
+        </button>
+        <button
+          type="button"
+          class="btn btn-warning"
+          @click="choose('overwrite')"
+        >
+          Save anyway (overwrite)
+        </button>
+        <button
+          type="button"
+          class="btn btn-primary"
+          @click="choose('discard')"
+        >
+          Discard my edits, pull latest
+        </button>
+      </div>
+    </div>
+    <form method="dialog" class="modal-backdrop">
+      <button type="submit" @click="choose('cancel')">close</button>
+    </form>
+  </dialog>
+</template>