feat: add mailbox app (inbox, read, compose)

Three-page mailbox suite, restoring functionality from v3:

- mailbox/inbox.html    Folder rail (Inbox / Sent / Drafts / Starred /
                       Archive / Spam / Trash) with badge counts, label
                       chips, bulk-action toolbar, unread row state,
                       search.
- mailbox/read.html     Sender meta, message body, attachment chips,
                       reply / forward / archive / delete actions.
- mailbox/compose.html  To / Cc / Bcc / Subject / Body / Attachments
                       form. Hooks for plugging in a rich-text editor
                       are documented in the field hint.

Pure Bootstrap 5, no custom CSS, no external dependencies.