Dashboard: own-messages view with management actions #63

Part of #62 — Dashboard & data-management surface

The core of the Dashboard: the user's own messages, separate from the public feed, with the
management actions attached (edit, delete, share, create-from, select-mode for bulk actions).

• Backed by the onlyMine query parameter on GET /api/messages (see the feed epic).
• A table on the web; on a phone this should be a dense list, not a literal table.
• This is also where multi-select for Create from… naturally lives.

Tests: query carries onlyMine; actions operate on the right message.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.