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>

static/js/scheduler.js

@@ -242,7 +242,8 @@ function schedLoadHistory() {
     var html = '';
     runs.forEach(function(r) {
       var ts = r.started_at ? new Date(r.started_at * 1000).toLocaleString() : '-';
-      var icon = r.status === 'completed' ? '\u2713' : r.status === 'failed' ? '\u2716' : '\u23f3';
+      var icon = r.status === 'completed' ? '\u2713' : r.status === 'failed' ? '\u2716'
+               : r.status === 'cancelled' ? '\u23f9' : '\u23f3';
       var jname = r.job_name ? '<strong>' + _esc(r.job_name) + '</strong> - ' : '';
       html += icon + ' ' + jname + ts + ' - ' + (r.flagged||0) + ' flagged';
       if (r.emailed) html += ' \u2709';