feat: scan history browser, user-scoped viewer tokens, export fixes, email fixes (v1.6.20)

- Scan history browser (history.js, GET /api/db/sessions, get_sessions(),
  get_session_items(ref_scan_id)) — review any past session without rescanning
- User-scoped viewer tokens (#34) — scope by individual employee across M365
  and GWS; autocomplete from Accounts list; dual-email support
- Fix: GWS scan never marked finished (end_scan → finish_scan) and emitted
  wrong SSE event (scan_done → google_scan_done), excluding GWS items from all
  exports
- Fix: file scan begin_scan called with wrong keyword args (TypeError swallowed),
  so local/SMB items were never written to DB
- Fix: Graph sendMail reported failure on success — _post() now returns {} on
  empty 202 response instead of raising JSONDecodeError
- Fix: Graph error hidden behind generic "No SMTP host" message when both Graph
  and SMTP were unavailable
- Fix: Gmail vs Google Workspace SMTP error messages distinguished by username
  domain; Workspace errors point to admin console, not personal security settings
- Docs: update README, MANUAL-EN, MANUAL-DA, CLAUDE.md, TODO.md, CHANGELOG.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

m365_connector.py

@@ -473,7 +473,7 @@ class M365Connector:
                     msg = r.text[:200]
                 raise M365PermissionError(path, msg)
             r.raise_for_status()
-            return r.json()
+            return r.json() if r.content else {}
         raise _requests.exceptions.RetryError(f"Gave up after {self._MAX_RETRIES} attempts: {url}")
 
     def _get_bytes(self, url: str, _retry: bool = True) -> bytes: