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>

lang/en.json

@@ -260,6 +260,10 @@
   "m365_admin_note": "Only showing your account. To list all users, an admin must grant <strong>User.Read.All</strong> consent in Azure Portal, or add accounts manually below.",
   "m365_btn_scan": "Scan",
   "m365_btn_stop": "Stop",
+  "m365_btn_stopping": "Stopping…",
+  "m365_scan_stopping": "Stopping scan — finishing the current item…",
+  "m365_scan_stopped": "Scan stopped.",
+  "m365_sched_stopped": "stopped",
   "m365_pill_flagged": "flagged",
   "m365_pill_scanned": "scanned",
   "m365_filter_all_sources": "All sources",