Keep Google-only accounts out of the M365 scan

Graph returned 404 for /users/google:<email>/mailFolders: Google-
Workspace-only accounts (id "google:<email>") reached the M365 scan.

- getSelectedUsers() fell back to "every ticked checkbox in the DOM"
  whenever no M365 account was ticked — including Google-only rows. The
  fallback is now only used before the user list has loaded, and never
  includes google: rows.
- startScan() skips the M365 part (with a log line) when M365 sources are
  ticked but no M365 accounts are: the server treats an empty account
  list as "every user in the tenant".
- run_scan: _resolve_manual_users -> _resolve_scan_users, which now also
  drops google: ids (reported as a phase message). Covers scheduled scans
  and profiles that already contain Google-only accounts. Never widens to
  all users.

Tests: 3 new in tests/test_manual_users.py; the run_scan one fails on the
old engine with exactly the reported Graph call.

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

lang/en.json

@@ -260,6 +260,7 @@
   "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_no_m365_accounts": "No Microsoft 365 accounts selected — skipping the Microsoft 365 sources.",
   "m365_btn_stopping": "Stopping…",
   "m365_scan_stopping": "Stopping scan — finishing the current item…",
   "m365_scan_stopped": "Scan stopped.",
@@ -517,6 +518,7 @@
   "m365_filter_special_only": "Art. 9 special category",
   "m365_badge_special": "Art.9",
   "m365_phase_scanning": "Scanning…",
+  "m365_skipped_google_accounts": "Skipped {n} Google Workspace-only account(s) — not in Microsoft 365",
   "a30_special_cat": "Art. 9 special category items",
   "a30_special_cat_note": "These items contain health, criminal, biometric, religious, ethnic, trade union, political, or sexual orientation data. An explicit legal basis (Art. 9(2)) and possibly a DPIA (Art. 35) is required.",
   "a30_col_special": "Art. 9",