Add step-by-step Microsoft app registration guide for Mail

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

README.md

@@ -78,7 +78,7 @@ e.g. hub.necloud.us. Settings: `SEAFILE_URL`, `ONLYOFFICE_URL` env vars in docke
 - **Podcast** and **Podcast Admin**: launchers that open `PODCAST_URL` (a home-network address) and `VERSE_URL` in their own tabs.
 - **Office**: embeds onlyoffice.necloud.us in a frame, with an "open in new tab" fallback if it blocks framing.
 - **Mail**: your **Outlook.com** (personal) mailbox inside the app: folders with unread counts, message list with search, reader, compose with attachments (up to about 3 MB), reply, reply all, forward, archive, delete, flag, mark unread, an unread badge on the Mail tab and a Home card. It uses Microsoft Graph, because Outlook.com no longer allows IMAP passwords. Each person connects their own mailbox (Mail tab -> Connect Outlook.com, sign-in is on Microsoft's own page); the hub keeps only the sign-in token, encrypted, in that person's folder, and everything is private to them. Message text is shown in a sandboxed frame with no scripts, and pictures from the internet stay hidden until you press Show pictures. Not included: drafts autosave, contacts, several mailboxes per person, Microsoft 365 work accounts, Gmail/iCloud (those can be added the same way later).
-  **One-time setup** (needs a free Microsoft account; takes five minutes): go to entra.microsoft.com -> App registrations -> New registration. Name it NE Hub; supported account types **Personal Microsoft accounts only**; Redirect URI: platform **Web**, value `<your hub address>/api/mail/callback` (for example `https://hub.necloud.us/api/mail/callback`; it must match `PUBLIC_URL` exactly). Then API permissions -> Add -> Microsoft Graph -> Delegated: `Mail.ReadWrite`, `Mail.Send`, `User.Read`, `offline_access`. Copy the **Application (client) ID** into `MS_CLIENT_ID`. If you registered it as a Web app, also make a client secret (Certificates & secrets) and put its value in `MS_CLIENT_SECRET`; if you used platform "Mobile and desktop applications" instead, no secret is needed. Leave `MS_CLIENT_ID` empty to switch Mail off. (The old bundled Cypht webmail and its `nehub-cypht` volume are gone; you can delete that volume.) Connecting from the installed iPhone/iPad app may open Microsoft's page in Safari; if the app does not notice, reopen it.
+  **One-time setup** (step by step, with troubleshooting, in [docs/mail-setup.md](docs/mail-setup.md); needs a free Microsoft account and takes five minutes): go to entra.microsoft.com -> App registrations -> New registration. Name it NE Hub; supported account types **Personal Microsoft accounts only**; Redirect URI: platform **Web**, value `<your hub address>/api/mail/callback` (for example `https://hub.necloud.us/api/mail/callback`; it must match `PUBLIC_URL` exactly). Then API permissions -> Add -> Microsoft Graph -> Delegated: `Mail.ReadWrite`, `Mail.Send`, `User.Read`, `offline_access`. Copy the **Application (client) ID** into `MS_CLIENT_ID`. If you registered it as a Web app, also make a client secret (Certificates & secrets) and put its value in `MS_CLIENT_SECRET`; if you used platform "Mobile and desktop applications" instead, no secret is needed. Leave `MS_CLIENT_ID` empty to switch Mail off. (The old bundled Cypht webmail and its `nehub-cypht` volume are gone; you can delete that volume.) Connecting from the installed iPhone/iPad app may open Microsoft's page in Safari; if the app does not notice, reopen it.
 
 ## Install on iPhone / iPad (PWA)