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>

gdpr_scanner.py

@@ -1527,16 +1527,9 @@ def local_ip():
 def scan_stream():
     q = queue.Queue(maxsize=512)
     _sse_queues.append(q)
-    # Filter replay buffer: only include events from the current scan
-    # (avoids replaying stale events from a previous scan)
-    replay_scan_id = _sse_mod._current_scan_id
-    buf = []
-    if replay_scan_id:
-        for msg in list(_sse_buffer):
-            if f'"scan_id": "{replay_scan_id}"' in msg:
-                buf.append(msg)
-    else:
-        buf = list(_sse_buffer)
+    # Replay only the running scan's events, never those of a finished or
+    # stopped scan (see sse.replay_messages)
+    buf = _sse_mod.replay_messages()
     def generate():
         try:
             yield ": connected\n\n"