Slack module: several channels, send tiles follow the transcript clicked last

One Slack tile per channel instead of one channel per machine. Admin picks
any number of the account's channels (`channels`, a multiselect fed by
/channels; a `channel` string saved by 1.0.0 is read as a one-item list, the
module never rewrites admin config). tiles() offers one transcript entry per
channel — id "transcript:<id>", named "#<channel>" so the shell titles the
tile with it, settings { view, channel } — plus Send message and Quick
replies, each `single: true`. A transcript tile from 1.0.0 (no channel in its
settings) shows the first configured channel.

Server: one poller per channel, each with its own window, thread cache,
membership and error, sharing the token, the name caches and the SSE fan-out.
Slack allows an internal app ~50+ conversations.history calls a minute per
token, so the module keeps itself under 40:

  interval per channel = max(pollSeconds, ceil(channels × 60 / 40)) s
  1 → 2 s, 2 → 3 s, 3 → 5 s, 5 → 8 s

with pollers phase-locked across that interval so they never fire in a
burst (first reads staggered 250 ms apart after boot), one auth.test shared
by all of them, and a 429 backing every poller off together. /state returns
`channels` keyed by id (name, isMember, messages, pollAge, error, status)
plus `channelOrder`; /stream stays one `state` snapshot per change. The
health line lists the channels and names any with a problem:
"Apollo Beach · #production, #ops · every 3 s (rate budget) · sending on".
/send and /quick take `channel`, which must be one of the configured ids
(400 otherwise) — a PIN-unlocked browser chooses among the admin's channels,
never an arbitrary one; a quick reply's own "| #channel" wins over it.

Client: a click anywhere in a transcript's body makes its channel the send
target; the chosen transcript carries a 3px accent stripe along the bottom
of the tile body (the chrome is the shell's), the others hint at it on hover.
The selection is one fact about the whole page, so it lives in a deliberate
module-level store in client.js (an EventTarget plus the id, mirrored in
localStorage under proddash:slack:sendTarget so a reload keeps it) — the
comment there says why this is not the per-instance state the guide forbids.
Send message and Quick replies show "Sending as Apollo Beach → #ops" and
follow the selection live; when the selected channel is no longer configured
or its transcript tile is gone, sends fall back to the first configured
channel, and the headers say so. PIN pad, lockout, unlock tokens, Enter to
send, auto-lock, the transcript rendering and the per-tile settings are as
they were.

The mock gains a quiet #ops trickle so two transcripts visibly move at their
own pace; a post to either channel already lands in that channel's history.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

modules/slack/style.css

@@ -30,6 +30,29 @@
   border-bottom-color: color-mix(in srgb, var(--danger) 35%, var(--border));
 }
 
+/* ── the send target ── */
+
+/* A click anywhere in a transcript's body makes its channel the one Send
+   message and Quick replies post to. The chosen one carries a 3px accent
+   stripe along the bottom edge of the tile body — drawn just inside our
+   root, since the tile chrome around it belongs to the shell. The others
+   hint at it faintly on hover. */
+[data-module="slack"] .sl-transcript::after {
+  content: "";
+  position: absolute;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  height: 3px;
+  background: var(--accent);
+  opacity: 0;
+  pointer-events: none;
+  z-index: 3;
+  transition: opacity 0.15s ease;
+}
+[data-module="slack"] .sl-transcript:hover::after { opacity: 0.3; }
+[data-module="slack"] .sl-transcript.is-target::after { opacity: 1; }
+
 /* ── transcript stream ── */
 
 [data-module="slack"] .sl-stream {