Show delta token source count, add hint bubble, fix README data paths

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lang/en.json

@@ -348,8 +348,9 @@
   "m365_resuming": "Resuming — skipping already-scanned items…",
   "m365_opt_delta": "Delta scan",
   "m365_opt_delta_hint": "Changed items only (after first full scan)",
-  "m365_delta_tokens_saved": "Tokens saved",
+  "m365_delta_tokens_saved": "Tokens saved for {n} source(s)",
   "m365_delta_clear": "Clear tokens",
+  "m365_delta_tokens_hint": "Saved change-tokens let delta scans fetch only items modified since the last scan. Clear tokens forces the next scan to be a full scan.",
   "m365_delta_cleared": "Delta tokens cleared — next scan will be a full scan.",
   "m365_delta_mode": "Delta mode — fetching changed items only…",
   "m365_smtp_title": "✉ Email report",