feat(chat): forward messages with multi-select (#1)

- Message action pill gains a Forward button; tapping it enters selection mode
  (tap bubbles to multi-select, footer bar shows count + Forward/Cancel, Esc exits).
- Forward picker lists EXISTING conversations only (DMs + groups from the sidebar),
  searchable, multi-target. POST /api/messages/forward copies body+attachment into
  each target (authorized as participant/member), live-pushed like a normal send.
- /files auth now accepts ANY message carrying an attachment (allByAttachment), so
  forwarded images stay viewable for the new recipients.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/repos.js

@@ -194,6 +194,7 @@ const messages = {
       .run(id, teamId, senderId, recipientId || '', body, now(), replyTo || null, attachmentId || null, conversationId || null, (mentions && mentions.length) ? JSON.stringify(mentions) : null, msgType || null),
   byId: (id) => db.prepare('SELECT * FROM messages WHERE id=?').get(id),
   byAttachment: (attachmentId) => db.prepare('SELECT * FROM messages WHERE attachment_id=? LIMIT 1').get(attachmentId),
+  allByAttachment: (attachmentId) => db.prepare('SELECT sender_id, recipient_id, conversation_id FROM messages WHERE attachment_id=? AND deleted=0').all(attachmentId),
   setPoll: (messageId, pollId) => db.prepare('UPDATE messages SET poll_id=? WHERE id=?').run(pollId, messageId),
   markDelivered: (id) => db.prepare('UPDATE messages SET delivered_at=? WHERE id=? AND delivered_at IS NULL').run(now(), id),
   editBody: (id, body) => db.prepare('UPDATE messages SET body=?, edited_at=? WHERE id=?').run(body, now(), id),