Make Stop stop: no replay, no restart, immediate feedback

Pressing Stop could look like the scan restarting, and for scheduled jobs
it really did restart the Google stage.

- Scheduler: check the abort flag between stages and never clear the
  Google abort after a Stop (that started a fresh all-users Google scan).
  A stopped job is recorded as "cancelled" and skips email/retention;
  scheduler_done carries cancelled=true.
- SSE: scan_cancelled and file_scan_done now end the current scan like
  scan_done, and sse.replay_messages() never replays a finished scan.
  Previously a stopped M365 scan (and every file scan) stayed "current",
  so each reconnect replayed it and re-showed Stop + progress.
- Frontend: scan_cancelled clears the M365 running flag and only tears
  down the UI when no Google/file scan is still running; scheduler_done /
  scheduler_error clear all running flags.
- run_scan: Stop during collection now ends the scan (checkpoint, finish,
  scan_cancelled) instead of announcing scan_start / "Scanning…".
- Stop button: switches to a disabled "Stopping…" state on click; all
  show/hide goes through _setStopBtnDisplay(), which resets it when hidden
  or when a new scan starts.

Adds tests/test_stop_scan.py (10 tests).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scan_engine.py

@@ -553,12 +553,6 @@ def run_scan(options: dict):
         if rem < 3600: return f"{int(rem/60)}m"
         return f"{int(rem/3600)}h"
 
-    def _check_abort():
-        if _state._scan_abort.is_set():
-            broadcast("scan_cancelled", {"completed": completed})
-            return True
-        return False
-
     def _broadcast_card(item_meta: dict, cprs: list, pii_counts: dict | None = None):
         card = {
             "id":           item_meta.get("id", ""),
@@ -1089,6 +1083,26 @@ def run_scan(options: dict):
     if scanned_ids:
         work_items = [(k, m, f) for k, m, f in work_items if m.get("id", "") not in scanned_ids]
 
+    def _finish_aborted(done: int) -> None:
+        """Stop requested: checkpoint, finalise the DB scan, announce it."""
+        # Save checkpoint so scan can be resumed later
+        _save_checkpoint(ck_key, scanned_ids, _state.flagged_items, _state.scan_meta)
+        # Finalise the DB scan record so items found before the stop stay
+        # visible — the early return otherwise skips finish_scan below,
+        # stranding them (invisible to get_session_items / get_open_items).
+        if _db and _db_scan_id:
+            try:
+                _db.finish_scan(_db_scan_id, done)
+            except Exception as _e:
+                logger.error("[db] finish_scan (aborted) failed: %s", _e)
+        broadcast("scan_cancelled", {"completed": done - resumed_count})
+
+    # Stop pressed during collection: the per-user loops just break out, so
+    # stop here — announcing scan_start/"Scanning…" made Stop look like a restart.
+    if _state._scan_abort.is_set():
+        _finish_aborted(resumed_count)
+        return
+
     total = len(work_items)
     broadcast("scan_start", {
         "total": total + resumed_count,
@@ -1107,17 +1121,8 @@ def run_scan(options: dict):
     _items_since_save = 0
     idx = -1
     while _work_q:
-        if _check_abort():
-            # Save checkpoint so scan can be resumed later
-            _save_checkpoint(ck_key, scanned_ids, _state.flagged_items, _state.scan_meta)
-            # Finalise the DB scan record so items found before the stop stay
-            # visible — this early return otherwise skips finish_scan below,
-            # stranding them (invisible to get_session_items / get_open_items).
-            if _db and _db_scan_id:
-                try:
-                    _db.finish_scan(_db_scan_id, resumed_count + idx + 1)
-                except Exception as _e:
-                    logger.error("[db] finish_scan (aborted) failed: %s", _e)
+        if _state._scan_abort.is_set():
+            _finish_aborted(resumed_count + idx + 1)
             return
         idx += 1
         kind, meta, _ = _work_q.popleft()  # releases this item from the deque immediately