Beeper composer shows an attach button that can never be enabled #10401

Problem

The Beeper chat composer always renders an Attach a file button, permanently disabled and titled "Attachments aren't supported yet". No tracker item covers sending attachments, so this is a control that can never work and has no plan behind it.

• Entry point: client/src/components/messages/beeper/BeeperThread.jsx (~L936–944): <button aria-label="Attach a file" title="Attachments aren't supported yet" disabled …>.
• The durable send outbox (apiBeeper.js, chore(deps): bump actions/setup-node from 4 to 6 #36) accepts text only, and no server route accepts an outgoing attachment.

Two comments also point readers to a TODO that doesn't exist:

• server/lib/db/schema/beeper.js (~L109): "see the TODO on markConversationSeen"
• client/src/components/messages/beeper/BeeperChatSurface.jsx (~L489): same reference

markConversationSeen (server/routes/beeper.js) carries no such TODO, so the deferred "also send a read receipt" toggle is tracked nowhere.

Decision: hide until it works

1. Remove the disabled attach button from BeeperThread.jsx and keep the composer layout intact, so textarea and send button stay aligned on mobile. Sending attachments can come back as its own feature once the outbox carries files.
2. Reword both comments to describe the read-receipt toggle as unbuilt, out-of-scope behavior, without pointing at a TODO that isn't there.

Acceptance criteria

• The Beeper composer shows no attach control.
• rg "see the TODO on" server client/src returns nothing for the Beeper files.
• Existing Beeper thread component tests still pass, updated if they assert on the attach button.

model:light / effort:low: a mechanical single-component removal plus two comment edits.