editor: collect the refund — the web editor autosaves on an idle pause

#2971's engine work was already done and its benefit was never taken up here.

Both engines coalesce revision snapshots to one per editing session —
`src/thoughtsync/revisions.py::should_snapshot` and `store.rs`'s namesake, the
server's applied on the PATCH path AND in `sync.py`, with four integration
tests covering it. So a write has cost a write, not a write plus a revision,
for some time.

But this editor still wrote only on `close()`. That save-on-close existed
BECAUSE writes were expensive; with the reason gone, all that was left was the
cost — a tab closed mid-paragraph lost the paragraph, which is the one thing a
notes app must not do. Android already debounces (`BoardViewModel`); the shared
Vue editor did not, so web and desktop kept paying for a trade that had been
cancelled.

Now: a 1s idle pause writes.

EDIT MODE ONLY, deliberately. In compose, `dismiss` discards a note that was
never persisted so an accidental keystroke or a type-to-compose never litters
the board. An autosave there would create the row and quietly take that
behaviour away. Materialising a compose on first keystroke is a separate
decision (#2967), not a side effect of this one.

Three details that decide whether it is safe rather than merely present:

  * `flush` returns without writing while a save is in flight, so an autosave
    landing there would silently drop everything typed since that save began.
    It RE-ARMS instead of skipping.
  * Errors are swallowed and retried on the next pause. An autosave that
    interrupts typing with a message is worse than one that waits, and `close`
    still surfaces a real failure where the person is looking.
  * The timer is cancelled by `close`, by `dismiss` and on unmount, so nothing
    fires through a component during its leave animation or after it is gone.

Checked and found harmless rather than assumed: `notes.reconcile` replaces the
store's item but never touches `useNoteEditor`'s `editing` ref, so the
`watch(() => props.note)` that calls `setBody` does not fire on a save. Were
that not true, autosaving would have reset the field and the caret every
second.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K3MMqUtzX1TJgA1oypvm1c

frontend/src/components/NoteEditor.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, nextTick, onMounted, ref, watch } from "vue";
+import { computed, nextTick, onBeforeUnmount, onMounted, ref, watch } from "vue";
 import { useNotesStore } from "../stores/notes";
 import Icon from "./Icon.vue";
 import LabelPicker from "./LabelPicker.vue";
@@ -165,6 +165,63 @@ async function flush(): Promise<void> {
   }
 }
 
+// ---- idle autosave ----
+//
+// This editor used to write ONLY on close, and the reason was cost: a body write
+// snapshotted a revision, so saving often meant a version history of thirty
+// snapshots of one paragraph being typed. The price was durability — a tab closed
+// mid-paragraph lost the paragraph, which is the one thing a notes app must not do.
+//
+// That trade is gone. Both engines now coalesce snapshots to one per editing
+// session (`revisions.py` and `store.rs`'s `should_snapshot`, Scribe #2971), so a
+// write costs a write. Writing on an idle pause is what collects the refund; the
+// Android editor already does the same.
+const AUTOSAVE_MS = 1000;
+let autosaveTimer: ReturnType<typeof setTimeout> | null = null;
+
+function cancelAutosave(): void {
+  if (autosaveTimer !== null) {
+    clearTimeout(autosaveTimer);
+    autosaveTimer = null;
+  }
+}
+
+async function autosave(): Promise<void> {
+  // `flush` returns without writing while a save is in flight, which would silently
+  // drop everything typed since that save began. Re-arming rather than skipping is
+  // what keeps that from being a lost paragraph.
+  if (saving.value) {
+    scheduleAutosave();
+    return;
+  }
+  try {
+    await flush();
+  } catch {
+    // Swallowed on purpose. An autosave that interrupts typing with an error is
+    // worse than one that waits for the next pause, and `close` still surfaces a
+    // real failure at the moment the person is looking at the editor.
+  }
+}
+
+function scheduleAutosave(): void {
+  cancelAutosave();
+  autosaveTimer = setTimeout(() => {
+    autosaveTimer = null;
+    void autosave();
+  }, AUTOSAVE_MS);
+}
+
+// EDIT mode only, deliberately. In compose, `dismiss` discards a note that was
+// never persisted, so that an accidental keystroke or a type-to-compose never
+// litters the board — and an autosave that created the row would take that away
+// without anyone asking for it. Materialising a compose on first keystroke is a
+// separate decision (Scribe #2967), not a side effect of this one.
+watch(body, () => {
+  if (!isCreate.value) scheduleAutosave();
+});
+
+onBeforeUnmount(cancelAutosave);
+
 function resetCompose(): void {
   noteId.value = null;
   setBody("");
@@ -225,6 +282,10 @@ async function finish(): Promise<void> {
 
 // Persist (create in compose, save in edit) and close the editor.
 async function close(): Promise<void> {
+  // Cancelled first: a timer that fires during the leave animation would write
+  // through a component on its way out, after `flush` has already saved the same
+  // text.
+  cancelAutosave();
   await flush();
   await finish();
 }
@@ -233,6 +294,7 @@ async function close(): Promise<void> {
 // commit it explicitly (Done, Ctrl/Cmd+Enter, or Shift+Enter). An existing note, or a
 // compose already persisted by a rich action, closes normally (saving its text).
 async function dismiss(): Promise<void> {
+  cancelAutosave();
   if (isCreate.value) {
     await finish();
     return;