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_scheduler.py

@@ -351,6 +351,9 @@ class ScanScheduler:
                 )
                 _fs_extra = {k: _profile_opts[k] for k in _FS_OPT_KEYS if k in _profile_opts}
                 for fs in options.get("file_sources", []):
+                    # Stop pressed during an earlier stage — don't start more stages
+                    if _m._scan_abort.is_set():
+                        break
                     # Resolve string IDs to full source dicts
                     if isinstance(fs, str):
                         fs = _all_file_sources.get(fs, {"path": fs, "label": fs})
@@ -374,7 +377,12 @@ class ScanScheduler:
                 # Legacy profiles store everything in sources[]
                 google_sources = [s for s in options.get("sources", [])
                                   if s in ("gmail", "gdrive")]
-            if google_sources and state.google_connector:
+            # POST /api/scan/stop sets both abort events. _scan_abort was cleared
+            # when this job started, so if it is set now the user pressed Stop
+            # during an earlier stage — the Google stage must not start, and its
+            # abort event must not be cleared (that turned Stop into a restart).
+            stopped = _m._scan_abort.is_set()
+            if google_sources and state.google_connector and not stopped:
                 from routes.google_scan import (
                     _run_google_scan as _rgs,
                     _scan_lock       as _gsl,
@@ -383,12 +391,15 @@ class ScanScheduler:
                 if _gsl.acquire(blocking=False):
                     try:
                         _gsa.clear()
-                        logger.info("[scheduler] Starting Google scan — sources=%s", google_sources)
-                        _rgs({
-                            "sources":     google_sources,
-                            "user_emails": [],  # empty → scan all workspace users
-                            "options":     options.get("options", {}),
-                        })
+                        # Stop sets _scan_abort first, so re-check after clearing:
+                        # a Stop landing in between must still win.
+                        if not _m._scan_abort.is_set():
+                            logger.info("[scheduler] Starting Google scan — sources=%s", google_sources)
+                            _rgs({
+                                "sources":     google_sources,
+                                "user_emails": [],  # empty → scan all workspace users
+                                "options":     options.get("options", {}),
+                            })
                     except Exception as _ge:
                         import traceback as _tb3
                         logger.error("[scheduler] Google scan error: %s\n%s", _ge, _tb3.format_exc())
@@ -399,18 +410,21 @@ class ScanScheduler:
 
             run["flagged"] = len(_m.flagged_items)
             run["scanned"] = _m.scan_meta.get("total_scanned", 0)
-            run["status"]  = "completed"
-            logger.info("[scheduler] Job '%s' completed — %d flagged, %d scanned",
-                        job_cfg.get("name", ""), run["flagged"], run["scanned"])
+            stopped = _m._scan_abort.is_set()
+            run["status"]  = "cancelled" if stopped else "completed"
+            logger.info("[scheduler] Job '%s' %s — %d flagged, %d scanned",
+                        job_cfg.get("name", ""), run["status"], run["flagged"], run["scanned"])
 
-            if job_cfg.get("auto_email") and state.flagged_items:
+            # A stopped job's results are partial — don't email them or apply
+            # retention based on them.
+            if not stopped and job_cfg.get("auto_email") and state.flagged_items:
                 try:
                     self._send_email_report(job_cfg)
                     run["emailed"] = 1
                 except Exception as e:
                     run["error"] = f"Scan OK, email failed: {e}"
 
-            if job_cfg.get("auto_retention") and job_cfg.get("retention_years"):
+            if not stopped and job_cfg.get("auto_retention") and job_cfg.get("retention_years"):
                 try:
                     self._run_retention(job_cfg)
                 except Exception as e:
@@ -420,6 +434,7 @@ class ScanScheduler:
             _m.broadcast("scheduler_done", {
                 "flagged": run["flagged"], "scanned": run["scanned"],
                 "emailed": run["emailed"], "job_name": job_cfg.get("name", ""),
+                "cancelled": stopped,
             })
 
         except Exception as e: