i18n: translate the chat composer and messages

Catching up on the fork's rule that every update also translates one existing
system — three updates went by without one.

The composer's placeholder takes the channel as a parameter rather than being
concatenated, since Portuguese puts the channel elsewhere in the sentence than
English does.

The translation hook is bound to `tr` in these two files, not `t`: both already
use `t` for transfer records in inner scopes, which shadowed the function and
made it uncallable exactly where it was needed.

docs/roadmap-resenha.md

@@ -72,6 +72,7 @@ fixas por `t('chave')`. O que faltar cai para o inglês sozinho.
 - Configurações → Privacidade
 - Configurações → Conexões (nasceu bilíngue)
 - Seletor de GIF
+- Chat: composer e mensagens (placeholder, anexos, responder, reações)
 - Registro de auditoria e Estatísticas (nasceram bilíngues)
 
 ### Fila sugerida de tradução